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.
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