Fixed time zone issue in tests

Use of datetime's fromtimestamp method in test_cache_request_unfresh_max_age is a problem since it honors local time zone. Then the http Date header forged is not UTC/GMT.
It makes this particular test fail for positive UTC offset time zone such as mine (France UTC+0100 winter time).

I also switched to time only and drop all datetime method as it is sometime confusing because of its naive/aware timezone property.

Also add GMT tag in TIME_FMT :
    http://tools.ietf.org/html/rfc2616#section-3.3