Adding If-Modified-Since HTTP Header when available.

I changed `-[AFHTTPClient HTTPRequestOperationWithRequest:success:failure:]` to include an `If-Modified-Since` HTTP header value in the `urlRequest` in case a cached response contains a `Last-Modified` HTTP header value as described in [14.25 If-Modified-Since](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html) or [here](http://nshipster.com/nsurlcache/).