MINOR: cache: Add Expires header value parsing

MINOR: cache: Add Expires header value parsing

When no Cache-Control max-age or s-maxage information is present in a
cached response, we need to parse the Expires header value (RFC 7234#5.3).
An invalid Expires date value or a date earlier than the reception date
will make the cache_entry stale upon creation.
For now, the Cache-Control and Expires headers are parsed after the
insertion of the response in the cache so even if the parsing of the
Expires results in an already stale entry, the entry will exist in the
cache.