fix caching of resources in HTTP backend

we use the "max-age" directive and the "Expires" header for calculating freshness only currently (see commit 2f10f91596b12b5b0c7d57b4575ea4e3b232ee0a ), since the freshness calculation was moved from cache-validating to the cache-saving code. We should calculate the freshness as described in http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2.3 and then when validating check whether the resource is still fresh.