"HTTP/1.1 specification defines a "freshness lifetime" for a response if a "Last-Modified" is present.
http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.2.4
> Also, if the response does have a Last-Modified time, the heuristic expiration value SHOULD be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
> The calculation to determine if a response has expired is quite simple:
response_is_fresh = (freshness_lifetime > current_age)
Does NSURLSession support it? If so, can the modifier, i.e. 10%, be adjusted?
