[OskarEichler]

Summary

Normalize membership keys and check the currently stored header entry.

Reproduction and verification

Headers.new(user_agent: 'demo').key?(:user_agent) is false before the fix. After delete_if removes the entry, key?('user-agent') incorrectly remains true. The fix corrects both; all four predicate aliases and nil/false values are covered.

• Individual patch: 24 focused Faraday checks passed (membership); all 639 existing RSpec examples pass; changed-file RuboCop and Ruby syntax pass.
• Combined installed-release-based branch: 639 existing examples, 1,124 focused checks, full RuboCop (75 files), YARD checks and gem packaging pass.
• External faraday-net_http adapter suite: 386 existing examples, zero failures with the combined fixes.
• Ruby 4.0.6 through rbenv. No production or live external HTTP operations.

Limitations

No test/spec files were added or changed because the originating repository explicitly forbids this. Reproductions ran outside the repository. This differs from Faraday's requested regression-test contribution convention and is disclosed for maintainer review. Other Ruby versions/platforms were not run locally. This PR includes only the focused source change; the other fixes and the consumer's separately verified merged JSON decoder patch #1687 are not added here.

Breaking-change notes

Predicates now agree with current normalized entries. This does not redesign all inherited Hash mutation APIs.