Decode form query keys consistently with values - #607

Summary

Decode form-style '+' consistently in query keys and values, while leaving encoded %2B as a literal plus. Use normalized scheme comparison so HTTP/HTTPS casing does not change decoding. Other schemes retain literal plus behavior.

Reproduction

require 'addressable'
p Addressable::URI.parse('https://example.com?a+b=c+d').query_values
# Before: {a+b => c d}; after: {a b => c d}

Verification

• Ruby 4.0.6 through rbenv; existing suite: 1,433 examples / 0 failures / 5 existing pending with pure Ruby IDNA, 1,466 examples / 0 failures / 5 existing pending with idn-ruby 0.1.5 + libidn.
• 14 focused external assertions pass on this individual patch; the combined installed-release branch also passes all focused cases and both existing suites.
• Comparative RuboCop Lint: 15 existing offenses before and after; no new offense (line references move). Syntax and git diff --check pass.

Compatibility and limitations

Behavior correction: unescaped '+' in HTTP(S)/relative query keys now means a space, consistent with values. Use %2B for a literal plus. Uppercase HTTP(S) now follows the same rule; mailto and FTP remain unchanged.

No dependency/version/data updates. No repository tests were added or changed: the commissioning repository explicitly prohibits new/modified tests, so reproduction checks run outside this repository. Other Ruby versions/platforms were not run locally; upstream CI may require maintainer approval. The existing normalization proposal #589 and IDNA2008 proposal #496 are separate and unchanged.