Fix query_form regression for undef/valueless params (GH#133)

[oalders]

Closes #133

Problem

URI 5.19 (GH#65, commit 9ee8098) changed query_form so that:

• encoding an undef value produced a bare key with no =, and
• parsing a param with no = decoded to undef.

This broke downstream code such as HTTP::Request::Common (which passes undef to mean an empty value) and diverged from the WHATWG URL spec, where a param with no = decodes to an empty string. Maintainers in the issue thread confirmed the pre-5.19 behavior was correct.

Changes

• Restore pre-5.19 behavior in lib/URI/_query.pm: an undef value encodes as key=, and a param with no = decodes to an empty string rather than undef.
• Update existing assertions in t/query.t and t/old-base.t to expect the restored behavior.
• Add regression tests covering scalar undef, undef among multiple values, undef in a hash, and empty-string round-tripping.
• Add a Changes entry.

Testing

• Full suite passes: prove -lr t/ -> 1037 tests, 0 failures.
• Red-green verified: the new regression tests fail against the pre-fix source and pass with the fix.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.09%. Comparing base (e5edc02) to head (59103fe).

Files with missing lines	Patch %	Lines
lib/URI/_query.pm	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
- Coverage   87.09%   87.09%   -0.01%     
==========================================
  Files          64       64              
  Lines        2093     2092       -1     
  Branches      512      511       -1     
==========================================
- Hits         1823     1822       -1     
  Misses        109      109              
  Partials      161      161              

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