Preserve mixed-encoding multipart form data

[sjh9714]

Summary

Fixes #187.

Multipart requests containing both UTF-8 text and binary data can raise Encoding::CompatibilityError while building the body. Append buffered multipart pieces as binary strings, preserving their bytes without transcoding or changing the caller's strings.

Changes

• Keep buffered multipart headers and values compatible across encodings.
• Add a local HTTP round-trip regression for mixed-encoding streams and inline file values, including non-ASCII names, with both Content-Length and chunked requests.

Testing

On macOS with Ruby 4.0.6:

• bundle exec ruby -Ilib -Itest/lib -rhelper test/net/http/test_http.rb --name /test_set_form/ (8 tests)
• bundle exec rake test (209 tests)
• bundle exec rake rbs:test (29 tests)
• bundle exec rake build

The new regression raises the reported encoding error before the fix. The other Ruby versions and operating systems in CI were not run locally.