HTTP: MUST ignore a [revalidation] response with an older Date header.

HTTP: MUST ignore a [revalidation] response with an older Date header.

Before this patch, Squid violated the RFC 7234 section 4 MUST
requirement: "When more than one suitable response is stored, a cache
MUST use the most recent response (as determined by the Date header
field)." This problem may be damaging in cache hierarchies where
parent caches may have different responses. Trusting the older response
may lead to excessive IMS requests, cache thrashing and other problems.