Cover header quality comparisons.

[samuel-williams-shopify]

Summary

• Exercise quality-factor comparisons for Accept, Accept-Charset, Accept-Encoding, and Accept-Language values.
• Restore full line coverage for those header implementations.

Testing

• bundle exec bake test (774 tests, 1574 assertions)
• COVERAGE=PartialSummary bundle exec bake test (all affected header files report 100% coverage; known CRuby while-loop artifacts remain)