diff --git a/.rubocop.yml b/.rubocop.yml
index 82a8d4e8..e5ed0589 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -55,6 +55,9 @@ Style/EachWithObject:
 Style/Encoding:
   Enabled: false
 
+Style/EmptyCaseCondition:
+  Enabled: false
+
 Style/HashSyntax:
   EnforcedStyle: hash_rockets
 
diff --git a/Gemfile b/Gemfile
index 6beced55..288076c9 100644
--- a/Gemfile
+++ b/Gemfile
@@ -20,7 +20,7 @@ group :test do
   gem "coveralls"
   gem "simplecov", ">= 0.9"
   gem "json",      ">= 1.8.1"
-  gem "rubocop",   "=  0.39.0"
+  gem "rubocop",   "=  0.40.0"
   gem "rspec",     "~> 3.0"
   gem "rspec-its"
   gem "yardstick"
diff --git a/lib/http.rb b/lib/http.rb
index ab4a94c2..4e17d2ea 100644
--- a/lib/http.rb
+++ b/lib/http.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "http/parser"
 
 require "http/errors"
diff --git a/lib/http/content_type.rb b/lib/http/content_type.rb
index 056ca4b9..a7318861 100644
--- a/lib/http/content_type.rb
+++ b/lib/http/content_type.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module HTTP
   ContentType = Struct.new(:mime_type, :charset) do
     MIME_TYPE_RE = %r{^([^/]+/[^;]+)(?:$|;)}
diff --git a/lib/http/errors.rb b/lib/http/errors.rb
index 199fff1e..8a1fc260 100644
--- a/lib/http/errors.rb
+++ b/lib/http/errors.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module HTTP
   # Generic error
   class Error < StandardError; end
diff --git a/lib/http/headers.rb b/lib/http/headers.rb
index 97229182..fce7027f 100644
--- a/lib/http/headers.rb
+++ b/lib/http/headers.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "forwardable"
 
 require "http/errors"
diff --git a/lib/http/headers/mixin.rb b/lib/http/headers/mixin.rb
index 1a947002..9aa95136 100644
--- a/lib/http/headers/mixin.rb
+++ b/lib/http/headers/mixin.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "forwardable"
 
 module HTTP
diff --git a/lib/http/mime_type.rb b/lib/http/mime_type.rb
index 3cefba1a..47a1f145 100644
--- a/lib/http/mime_type.rb
+++ b/lib/http/mime_type.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module HTTP
   # MIME type encode/decode adapters
   module MimeType
diff --git a/lib/http/mime_type/adapter.rb b/lib/http/mime_type/adapter.rb
index ed6c4989..14668969 100644
--- a/lib/http/mime_type/adapter.rb
+++ b/lib/http/mime_type/adapter.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "forwardable"
 require "singleton"
 
diff --git a/lib/http/mime_type/json.rb b/lib/http/mime_type/json.rb
index 1e2cf2d6..f2d56cde 100644
--- a/lib/http/mime_type/json.rb
+++ b/lib/http/mime_type/json.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "json"
 require "http/mime_type/adapter"
 
diff --git a/lib/http/options.rb b/lib/http/options.rb
index 1b2e1c45..a69834c3 100644
--- a/lib/http/options.rb
+++ b/lib/http/options.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "http/headers"
 require "openssl"
 require "socket"
diff --git a/lib/http/redirector.rb b/lib/http/redirector.rb
index 783273ea..86028ec8 100644
--- a/lib/http/redirector.rb
+++ b/lib/http/redirector.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "set"
 
 require "http/headers"
diff --git a/lib/http/response.rb b/lib/http/response.rb
index 44969f09..f49b7fc8 100644
--- a/lib/http/response.rb
+++ b/lib/http/response.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "forwardable"
 
 require "http/headers"
diff --git a/lib/http/response/body.rb b/lib/http/response/body.rb
index 89c0cec5..79404eff 100644
--- a/lib/http/response/body.rb
+++ b/lib/http/response/body.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "forwardable"
 require "http/client"
 
@@ -43,8 +44,9 @@ def to_s
         end
 
         begin
-          @streaming = false
-          @contents = "".force_encoding(encoding)
+          @streaming  = false
+          @contents   = String.new("").force_encoding(encoding)
+
           while (chunk = @client.readpartial)
             @contents << chunk.force_encoding(encoding)
           end
diff --git a/lib/http/response/parser.rb b/lib/http/response/parser.rb
index 3c7e1d58..38b1925a 100644
--- a/lib/http/response/parser.rb
+++ b/lib/http/response/parser.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module HTTP
   class Response
     class Parser
diff --git a/lib/http/response/status.rb b/lib/http/response/status.rb
index 73fb3444..7a68d94c 100644
--- a/lib/http/response/status.rb
+++ b/lib/http/response/status.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "delegate"
 
 require "http/response/status/reasons"
diff --git a/lib/http/response/status/reasons.rb b/lib/http/response/status/reasons.rb
index 119c53ba..ed2f8883 100644
--- a/lib/http/response/status/reasons.rb
+++ b/lib/http/response/status/reasons.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # AUTO-GENERATED FILE, DO NOT CHANGE IT MANUALLY
 
 require "delegate"
diff --git a/lib/http/timeout/global.rb b/lib/http/timeout/global.rb
index 01a41934..1a3731a8 100644
--- a/lib/http/timeout/global.rb
+++ b/lib/http/timeout/global.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "timeout"
 require "io/wait"
 
diff --git a/lib/http/timeout/null.rb b/lib/http/timeout/null.rb
index 5167251c..8d798807 100644
--- a/lib/http/timeout/null.rb
+++ b/lib/http/timeout/null.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "forwardable"
 require "io/wait"
 
diff --git a/lib/http/timeout/per_operation.rb b/lib/http/timeout/per_operation.rb
index 6d23b6cd..0c277fe1 100644
--- a/lib/http/timeout/per_operation.rb
+++ b/lib/http/timeout/per_operation.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "timeout"
 
 require "http/timeout/null"
diff --git a/spec/lib/http/client_spec.rb b/spec/lib/http/client_spec.rb
index f6dd46f9..7e1b6383 100644
--- a/spec/lib/http/client_spec.rb
+++ b/spec/lib/http/client_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # coding: utf-8
 
 require "support/http_handling_shared"
diff --git a/spec/lib/http/content_type_spec.rb b/spec/lib/http/content_type_spec.rb
index a5fc55ff..bb4da960 100644
--- a/spec/lib/http/content_type_spec.rb
+++ b/spec/lib/http/content_type_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::ContentType do
   describe ".parse" do
     context "with text/plain" do
diff --git a/spec/lib/http/headers/mixin_spec.rb b/spec/lib/http/headers/mixin_spec.rb
index c45b5634..95bde7c4 100644
--- a/spec/lib/http/headers/mixin_spec.rb
+++ b/spec/lib/http/headers/mixin_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Headers::Mixin do
   let :dummy_class do
     Class.new do
diff --git a/spec/lib/http/headers_spec.rb b/spec/lib/http/headers_spec.rb
index a71bf9a1..ab6fccd7 100644
--- a/spec/lib/http/headers_spec.rb
+++ b/spec/lib/http/headers_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Headers do
   subject(:headers) { described_class.new }
 
diff --git a/spec/lib/http/options/body_spec.rb b/spec/lib/http/options/body_spec.rb
index 5e437e47..ad4166aa 100644
--- a/spec/lib/http/options/body_spec.rb
+++ b/spec/lib/http/options/body_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Options, "body" do
   let(:opts) { HTTP::Options.new }
 
diff --git a/spec/lib/http/options/form_spec.rb b/spec/lib/http/options/form_spec.rb
index 1997958b..09237ae1 100644
--- a/spec/lib/http/options/form_spec.rb
+++ b/spec/lib/http/options/form_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Options, "form" do
   let(:opts) { HTTP::Options.new }
 
diff --git a/spec/lib/http/options/headers_spec.rb b/spec/lib/http/options/headers_spec.rb
index c2864e7f..2c3cae71 100644
--- a/spec/lib/http/options/headers_spec.rb
+++ b/spec/lib/http/options/headers_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Options, "headers" do
   let(:opts) { HTTP::Options.new }
 
diff --git a/spec/lib/http/options/json_spec.rb b/spec/lib/http/options/json_spec.rb
index 861ffa44..60e22342 100644
--- a/spec/lib/http/options/json_spec.rb
+++ b/spec/lib/http/options/json_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Options, "json" do
   let(:opts) { HTTP::Options.new }
 
diff --git a/spec/lib/http/options/merge_spec.rb b/spec/lib/http/options/merge_spec.rb
index d3703266..bc55c55e 100644
--- a/spec/lib/http/options/merge_spec.rb
+++ b/spec/lib/http/options/merge_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 
 RSpec.describe HTTP::Options, "merge" do
   let(:opts) { HTTP::Options.new }
@@ -23,7 +24,8 @@
       :body      => "body-foo",
       :json      => {:foo => "foo"},
       :headers   => {:accept => "json", :foo => "foo"},
-      :proxy     => {})
+      :proxy     => {}
+    )
 
     bar = HTTP::Options.new(
       :response   => :parsed_body,
@@ -36,27 +38,29 @@
       :headers            => {:accept => "xml", :bar => "bar"},
       :timeout_options    => {:foo => :bar},
       :ssl        => {:foo => "bar"},
-      :proxy      => {:proxy_address => "127.0.0.1", :proxy_port => 8080})
+      :proxy      => {:proxy_address => "127.0.0.1", :proxy_port => 8080}
+    )
 
     expect(foo.merge(bar).to_hash).to eq(
-      :response        => :parsed_body,
-      :timeout_class   => described_class.default_timeout_class,
-      :timeout_options => {:foo => :bar},
-      :params     => {:plop => "plip"},
-      :form       => {:bar => "bar"},
-      :body       => "body-bar",
-      :json       => {:bar => "bar"},
-      :persistent => "https://www.googe.com",
+      :response           => :parsed_body,
+      :timeout_class      => described_class.default_timeout_class,
+      :timeout_options    => {:foo => :bar},
+      :params             => {:plop => "plip"},
+      :form               => {:bar => "bar"},
+      :body               => "body-bar",
+      :json               => {:bar => "bar"},
+      :persistent         => "https://www.googe.com",
       :keep_alive_timeout => 10,
-      :ssl       => {:foo => "bar"},
-      :headers   => {"Foo" => "foo", "Accept" => "xml", "Bar" => "bar"},
-      :proxy     => {:proxy_address => "127.0.0.1", :proxy_port => 8080},
-      :follow => nil,
-      :socket_class     => described_class.default_socket_class,
-      :nodelay          => false,
-      :ssl_socket_class => described_class.default_ssl_socket_class,
-      :ssl_context      => nil,
-      :cookies          => {},
-      :encoding         => nil)
+      :ssl                => {:foo => "bar"},
+      :headers            => {"Foo" => "foo", "Accept" => "xml", "Bar" => "bar"},
+      :proxy              => {:proxy_address => "127.0.0.1", :proxy_port => 8080},
+      :follow             => nil,
+      :socket_class       => described_class.default_socket_class,
+      :nodelay            => false,
+      :ssl_socket_class   => described_class.default_ssl_socket_class,
+      :ssl_context        => nil,
+      :cookies            => {},
+      :encoding           => nil
+    )
   end
 end
diff --git a/spec/lib/http/options/new_spec.rb b/spec/lib/http/options/new_spec.rb
index 123545fd..eef76898 100644
--- a/spec/lib/http/options/new_spec.rb
+++ b/spec/lib/http/options/new_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Options, "new" do
   it "supports a Options instance" do
     opts = HTTP::Options.new
diff --git a/spec/lib/http/options/proxy_spec.rb b/spec/lib/http/options/proxy_spec.rb
index 41fafd9f..b120a202 100644
--- a/spec/lib/http/options/proxy_spec.rb
+++ b/spec/lib/http/options/proxy_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Options, "proxy" do
   let(:opts) { HTTP::Options.new }
 
diff --git a/spec/lib/http/options_spec.rb b/spec/lib/http/options_spec.rb
index 1ab21920..c901ef50 100644
--- a/spec/lib/http/options_spec.rb
+++ b/spec/lib/http/options_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Options do
   subject { described_class.new(:response => :body) }
 
diff --git a/spec/lib/http/redirector_spec.rb b/spec/lib/http/redirector_spec.rb
index 0c1ba3ac..ab966c3d 100644
--- a/spec/lib/http/redirector_spec.rb
+++ b/spec/lib/http/redirector_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Redirector do
   def simple_response(status, body = "", headers = {})
     HTTP::Response.new(
diff --git a/spec/lib/http/request/writer_spec.rb b/spec/lib/http/request/writer_spec.rb
index a90de13b..ac7ed1f8 100644
--- a/spec/lib/http/request/writer_spec.rb
+++ b/spec/lib/http/request/writer_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # coding: utf-8
 
 RSpec.describe HTTP::Request::Writer do
diff --git a/spec/lib/http/request_spec.rb b/spec/lib/http/request_spec.rb
index dcba943c..40d38b77 100644
--- a/spec/lib/http/request_spec.rb
+++ b/spec/lib/http/request_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # coding: utf-8
 
 RSpec.describe HTTP::Request do
diff --git a/spec/lib/http/response/body_spec.rb b/spec/lib/http/response/body_spec.rb
index 6e1488d0..4fca7a70 100644
--- a/spec/lib/http/response/body_spec.rb
+++ b/spec/lib/http/response/body_spec.rb
@@ -1,17 +1,18 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Response::Body do
   let(:client)   { double(:sequence_id => 0) }
-  let(:chunks)   { ["Hello, ", "World!"] }
+  let(:chunks)   { [String.new("Hello, "), String.new("World!")] }
 
   before         { allow(client).to receive(:readpartial) { chunks.shift } }
 
-  subject(:body) { described_class.new client, Encoding::UTF_8 }
+  subject(:body) { described_class.new(client, Encoding::UTF_8) }
 
   it "streams bodies from responses" do
-    expect(subject.to_s).to eq "Hello, World!"
+    expect(subject.to_s).to eq("Hello, World!")
   end
 
   context "when body empty" do
-    let(:chunks) { [""] }
+    let(:chunks) { [String.new("")] }
 
     it "returns responds to empty? with true" do
       expect(subject).to be_empty
diff --git a/spec/lib/http/response/status_spec.rb b/spec/lib/http/response/status_spec.rb
index 4db54939..b8a457a8 100644
--- a/spec/lib/http/response/status_spec.rb
+++ b/spec/lib/http/response/status_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Response::Status do
   describe ".new" do
     it "fails if given value does not respond to #to_i" do
diff --git a/spec/lib/http/response_spec.rb b/spec/lib/http/response_spec.rb
index cb2c5781..2842ba4b 100644
--- a/spec/lib/http/response_spec.rb
+++ b/spec/lib/http/response_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::Response do
   let(:body)          { "Hello world!" }
   let(:uri)           { "http://example.com/" }
diff --git a/spec/lib/http/uri_spec.rb b/spec/lib/http/uri_spec.rb
index ab9111f5..f402bbde 100644
--- a/spec/lib/http/uri_spec.rb
+++ b/spec/lib/http/uri_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.describe HTTP::URI do
   let(:example_http_uri_string)  { "http://example.com" }
   let(:example_https_uri_string) { "https://example.com" }
diff --git a/spec/lib/http_spec.rb b/spec/lib/http_spec.rb
index a58aa959..5765c1d7 100644
--- a/spec/lib/http_spec.rb
+++ b/spec/lib/http_spec.rb
@@ -1,4 +1,5 @@
-# encoding: UTF-8
+# frozen_string_literal: true
+# encoding: utf-8
 
 require "json"
 
diff --git a/spec/regression_specs.rb b/spec/regression_specs.rb
index ac84f86d..bb4af170 100644
--- a/spec/regression_specs.rb
+++ b/spec/regression_specs.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "spec_helper"
 
 RSpec.describe "Regression testing" do
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index 4c4e4eee..328f7106 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # coding: utf-8
 
 require "simplecov"
diff --git a/spec/support/black_hole.rb b/spec/support/black_hole.rb
index a9df555b..38304a87 100644
--- a/spec/support/black_hole.rb
+++ b/spec/support/black_hole.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module BlackHole
   def self.method_missing(*)
     self
diff --git a/spec/support/capture_warning.rb b/spec/support/capture_warning.rb
index c39da4c6..502f9417 100644
--- a/spec/support/capture_warning.rb
+++ b/spec/support/capture_warning.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 def capture_warning
   old_stderr = $stderr
   $stderr = StringIO.new
diff --git a/spec/support/dummy_server.rb b/spec/support/dummy_server.rb
index 6aee8417..671fbeb3 100644
--- a/spec/support/dummy_server.rb
+++ b/spec/support/dummy_server.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "webrick"
 require "webrick/ssl"
 
diff --git a/spec/support/dummy_server/servlet.rb b/spec/support/dummy_server/servlet.rb
index 55fb30f1..d09abbc4 100644
--- a/spec/support/dummy_server/servlet.rb
+++ b/spec/support/dummy_server/servlet.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # encoding: UTF-8
 
 class DummyServer < WEBrick::HTTPServer
diff --git a/spec/support/http_handling_shared.rb b/spec/support/http_handling_shared.rb
index 3d188d81..4a239629 100644
--- a/spec/support/http_handling_shared.rb
+++ b/spec/support/http_handling_shared.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 RSpec.shared_context "HTTP handling" do
   describe "timeouts" do
     let(:conn_timeout) { 1 }
diff --git a/spec/support/proxy_server.rb b/spec/support/proxy_server.rb
index b268291a..a3886340 100644
--- a/spec/support/proxy_server.rb
+++ b/spec/support/proxy_server.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "webrick/httpproxy"
 
 require "support/black_hole"
diff --git a/spec/support/servers/config.rb b/spec/support/servers/config.rb
index 527a5d51..0630b42e 100644
--- a/spec/support/servers/config.rb
+++ b/spec/support/servers/config.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module ServerConfig
   def addr
     config[:BindAddress]
diff --git a/spec/support/servers/runner.rb b/spec/support/servers/runner.rb
index 6c9f1100..0e0170e7 100644
--- a/spec/support/servers/runner.rb
+++ b/spec/support/servers/runner.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module ServerRunner
   def run_server(name)
     let! name do
diff --git a/spec/support/ssl_helper.rb b/spec/support/ssl_helper.rb
index 18945360..afb16bc8 100644
--- a/spec/support/ssl_helper.rb
+++ b/spec/support/ssl_helper.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require "pathname"
 
 require "certificate_authority"