HTTP Compliance: entry is stale if request has max-age=0

HTTP Compliance: entry is stale if request has max-age=0.

We should always do validation for requests with Cache-Control
max-age=0, even when entry age is also zero. In our case, RFC 2616 says:

     freshness_lifetime = max_age_value
     response_is_fresh = (freshness_lifetime > current_age)

response_is_fresh is always false if freshness_lifetime is zero...

The check code was introduced in r5998 with a "Import of fix-ranges
branch" message. The code was commented out at the time of that commit,
for reasons unknown.

Test case:
     test_case/rfc2616/noSrv-hit-stale-max-age-req