HTTP cache: respect Age header in response

... which can be added to a HTTP response by intermediate servers. E.g. currently when receiving an HTTP response with "Cache-Control: max-age=60" and "Age: 50", we would still cache it for 60 seconds instead of 10 seconds. See http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2.3