Fix: Referer and HEAD using WWW::Mechanize (GH#150)

[oalders]

Closes #150

Problem

A $mech->head($url) request leaked its URL as the Referer header on subsequent get() requests, even after calling back().

_update_page() set $self->{last_uri} for every successful response, and last_uri is the sole source of the Referer header in _modify_request(). Because a HEAD request does not push the page stack, a following back() had nothing to pop, so the HEAD URL persisted as last_uri and leaked as the Referer of the next request.

Changes

• lib/WWW/Mechanize.pm: gate the last_uri update in _update_page() on the request being GET or POST — the same condition request() uses to decide whether to push the page stack. Non-navigational requests (HEAD/PUT/DELETE) no longer affect the Referer sent on subsequent requests.
• t/local/referer.t: add coverage that a HEAD does not clobber the Referer of the next request, including the back() case called out in the issue. Converted the fixed tests => N plan to done_testing.
• t/local/referer-server: raise the connection ceiling to cover the new requests, with an explanatory comment.
• Changes: note the fix.

Testing

• New regression tests fail without the fix and pass with it (red-green verified).
• Full t/local/ suite passes (322 tests).
• Edited .pm/.t files pass perltidy --assert-tidy.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.81%. Comparing base (93c8115) to head (088f163).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
+ Coverage   89.80%   89.81%   +0.01%     
==========================================
  Files           3        3              
  Lines         853      854       +1     
  Branches      225      226       +1     
==========================================
+ Hits          766      767       +1     
  Misses         37       37              
  Partials       50       50              

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[oalders]

@petdance I saw you had linked a branch for a similar fix, but it didn't appear to have any new commits in it. Does this fix seem reasonable?