Fix Ruby 3.4 frozen string literal warnings with StringIO

lib/marcel/magic.rb

@@ -115,7 +115,11 @@ 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)
+
+      # Make StringIO readonly before encoding changes to prevent Ruby 3.4 frozen string warnings.
+      # Should be fixed in Ruby 3.5+: https://redmine.ruby-lang.org/issues/21280
+      io.close_write if io.respond_to?(:closed_write?) && !io.closed_write?
 
       io.binmode if io.respond_to?(:binmode)
       io.set_encoding(Encoding::BINARY) if io.respond_to?(:set_encoding)

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