HTTP Compliance: Make date parser stricter to better handle malformed Expires.

HTTP Compliance: Make date parser stricter to better handle malformed Expires.

Check that there is no zone or zone is "GMT" in parse_date_elements().
Make sure there is no junk at the end of date in parse_date().

This will affect Date, IMS, and other date-carrying header fields recognized
by Squid but should not cause any messages to be rejected. Squid would just
ignore the malformed headers as if they are not there.

Co-Advisor test case:
    test_case/rfc2616/invalidExpiresMakesStale-rfc1123x