Fix Ruby 3.4 frozen string literal warnings with StringIO

PR #123 fixed frozen string warnings on line 127 (buffer creation)
but missed the warnings triggered by calling binmode/set_encoding on
StringIO objects at lines 120-121.

These methods modify StringIO's internal string encoding, which
triggers Ruby 3.4 deprecation warnings when running with verbose
warnings (-W2), which Rails test suites use by default.

Skip these calls for StringIO objects since marcel's byte-reading
operations work correctly without the encoding modification, as
demonstrated by all 384 tests passing.

This maintains the existing defensive respond_to? checks for File objects.

Author: andynu

lib/marcel/magic.rb

@@ -115,10 +115,13 @@ def self.child?(child, parent)
     end
 
     def self.magic_match(io, method)
-      return magic_match(StringIO.new(io.to_s), method) unless io.respond_to?(:read)
+      return magic_match(StringIO.new(+io.to_s), method) unless io.respond_to?(:read)
+
+      unless io.is_a?(StringIO)
+        io.binmode if io.respond_to?(:binmode)
+        io.set_encoding(Encoding::BINARY) if io.respond_to?(:set_encoding)
+      end
 
-      io.binmode if io.respond_to?(:binmode)
-      io.set_encoding(Encoding::BINARY) if io.respond_to?(:set_encoding)
       buffer = (+"").encode(Encoding::BINARY)
 
       MAGIC.send(method) { |type, matches| magic_match_io(io, matches, buffer) }

test/magic_test.rb

@@ -25,4 +25,25 @@ class Marcel::MimeType::MagicTest < Marcel::TestCase
     assert Marcel::Magic.child?('text/csv', 'text/plain')
     refute Marcel::Magic.child?('text/plain', 'text/csv')
   end
+
+  test "no Ruby 3.4 frozen string warnings with StringIO" do
+    # Ruby 3.4 warns about code that will break when frozen string literals become default
+    # This test ensures marcel handles StringIO with frozen strings correctly
+    content = "Test content for mime detection"
+    io = StringIO.new(content)
+
+    # Capture warnings
+    warnings = []
+    original_stderr = $stderr
+    $stderr = StringIO.new
+
+    begin
+      Marcel::MimeType.for(io)
+      warnings = $stderr.string.lines.grep(/marcel.*magic\.rb.*frozen/)
+    ensure
+      $stderr = original_stderr
+    end
+
+    assert_empty warnings, "Expected no frozen string warnings, but got:\n#{warnings.join}"
+  end
 end