HTTP cache: implement heuristics to calculate freshness of a resource

... as described in http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2.4 .
There is the possibility to cache resources that only have Last-Modified set (e.g. the RFC recommends to consider a resource as fresh for 10% of the time difference of now - LastModified), apparently implemented by Firefox, Chrome and Opera; apparently not implemented by Internet Explorer.