Fix Ruby 3.4 frozen string literal warnings with StringIO

[andynu]

Fixes #139

Problem

PR #123 fixed frozen string warnings on line 127, but missed warnings triggered by calling binmode and set_encoding on StringIO objects. Rails applications running tests with verbose warnings (-W2) see this warning repeatedly:

/gems/marcel-1.1.0/lib/marcel/magic.rb:120: warning: literal string will be frozen in the future

Solution

Make StringIO readonly before encoding modifications to prevent the warning:

# 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)

How it works:

• StringIO objects have a closed_write? method that File objects don't
• Calling close_write makes the StringIO readonly, preventing internal string modifications
• This prevents the frozen string warning when binmode/set_encoding are subsequently called
• File objects don't have closed_write?, so they're unaffected

Why this approach:

• Uses respond_to? pattern consistently with existing code
• still calls binmode and set_encoding as originally intended
• Proactively checks writability before calling (no rescue needed)
• Only affects StringIO, which is the only IO type that triggers these warnings

Testing

Added a test that captures stderr and verifies no frozen string warnings when processing StringIO objects. All 384 existing tests pass. The test provides a runnable example that demonstrates the fix works correctly.

Future

This workaround should only be needed until Ruby 3.5, which will include an upstream fix that prevents false-positive warnings on StringIO encoding operations: https://redmine.ruby-lang.org/issues/21280