lint: avoid TypeError on non-Hash-like response headers

According to SPEC (and check_headers), Response headers need only
respond to #each.  Thus, check_hijack_response should rely on
Rack::Utils::HeaderHash if it wishes to access the headers in a
hash-like fashion.
Added spec for new Squid behaviour.
Also see http://www.squid-cache.org/Doc/config/forwarded_for/
Merge pull request #499 from barttenbrinke/master

Added specific test when X-Forwarded-For is 'unknown'
Merge branch 'lint-headerhash' of git://bogomips.org/rack

* 'lint-headerhash' of git://bogomips.org/rack:
  lint: avoid TypeError on non-Hash-like response headers