diff --git a/lib/net/http.rb b/lib/net/http.rb
index 9e4f9e84..4cf3fa3e 100644
--- a/lib/net/http.rb
+++ b/lib/net/http.rb
@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 #
 # = net/http.rb
 #
@@ -1615,8 +1615,8 @@ def connect
                                       write_timeout: @write_timeout,
                                       continue_timeout: @continue_timeout,
                                       debug_output: @debug_output)
-          buf = "CONNECT #{conn_address}:#{@port} HTTP/#{HTTPVersion}\r\n"
-          buf << "Host: #{@address}:#{@port}\r\n"
+          buf = +"CONNECT #{conn_address}:#{@port} HTTP/#{HTTPVersion}\r\n" \
+            "Host: #{@address}:#{@port}\r\n"
           if proxy_user
             credential = ["#{proxy_user}:#{proxy_pass}"].pack('m0')
             buf << "Proxy-Authorization: Basic #{credential}\r\n"
diff --git a/lib/net/http/backward.rb b/lib/net/http/backward.rb
index 691e41e4..b44577ed 100644
--- a/lib/net/http/backward.rb
+++ b/lib/net/http/backward.rb
@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 # for backward compatibility
 
 # :enddoc:
diff --git a/lib/net/http/exceptions.rb b/lib/net/http/exceptions.rb
index 9c425cae..ceec8f7b 100644
--- a/lib/net/http/exceptions.rb
+++ b/lib/net/http/exceptions.rb
@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 module Net
   # Net::HTTP exception class.
   # You cannot use Net::HTTPExceptions directly; instead, you must use
diff --git a/lib/net/http/generic_request.rb b/lib/net/http/generic_request.rb
index 8877cd04..9421535c 100644
--- a/lib/net/http/generic_request.rb
+++ b/lib/net/http/generic_request.rb
@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 #
 # \HTTPGenericRequest is the parent of the Net::HTTPRequest class.
 #
@@ -316,7 +316,7 @@ def encode_multipart_form_data(out, params, opt)
     boundary ||= SecureRandom.urlsafe_base64(40)
     chunked_p = chunked?
 
-    buf = ''
+    buf = +''
     params.each do |key, value, h={}|
       key = quote_string(key, charset)
       filename =
@@ -401,7 +401,7 @@ def write_header(sock, ver, path)
     if /[\r\n]/ =~ reqline
       raise ArgumentError, "A Request-Line must not contain CR or LF"
     end
-    buf = ""
+    buf = +''
     buf << reqline << "\r\n"
     each_capitalized do |k,v|
       buf << "#{k}: #{v}\r\n"
diff --git a/lib/net/http/header.rb b/lib/net/http/header.rb
index e5fabec4..1d734a66 100644
--- a/lib/net/http/header.rb
+++ b/lib/net/http/header.rb
@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 #
 # The \HTTPHeader module provides access to \HTTP headers.
 #
diff --git a/lib/net/http/proxy_delta.rb b/lib/net/http/proxy_delta.rb
index a2f770eb..e7d30def 100644
--- a/lib/net/http/proxy_delta.rb
+++ b/lib/net/http/proxy_delta.rb
@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 module Net::HTTP::ProxyDelta   #:nodoc: internal use only
   private
 
diff --git a/lib/net/http/request.rb b/lib/net/http/request.rb
index e900b8a1..4a138572 100644
--- a/lib/net/http/request.rb
+++ b/lib/net/http/request.rb
@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 
 # This class is the base class for \Net::HTTP request classes.
 # The class should not be used directly;
diff --git a/lib/net/http/requests.rb b/lib/net/http/requests.rb
index 96cedcab..57241642 100644
--- a/lib/net/http/requests.rb
+++ b/lib/net/http/requests.rb
@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 
 # HTTP/1.1 methods --- RFC2616
 
diff --git a/lib/net/http/response.rb b/lib/net/http/response.rb
index 43a4a698..3a3156b0 100644
--- a/lib/net/http/response.rb
+++ b/lib/net/http/response.rb
@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 
 # This class is the base class for \Net::HTTP response classes.
 #
@@ -273,7 +273,7 @@ def code_type   #:nodoc:
 
   def error!   #:nodoc:
     message = @code
-    message += ' ' + @message.dump if @message
+    message = "#{message} #{@message.dump}" if @message
     raise error_type().new(message, self)
   end
 
@@ -648,7 +648,7 @@ def procdest(dest, block)
     if block
       Net::ReadAdapter.new(block)
     else
-      dest || ''
+      dest || +''
     end
   end
 
diff --git a/lib/net/https.rb b/lib/net/https.rb
index d46721c8..0f23e1fb 100644
--- a/lib/net/https.rb
+++ b/lib/net/https.rb
@@ -1,4 +1,4 @@
-# frozen_string_literal: false
+# frozen_string_literal: true
 =begin
 
 = net/https -- SSL/TLS enhancement for Net::HTTP.