AFHTTPRequestSerializer-requestWithMethod:URLString: should take NSURL object

[sebastianludwig]

I might be missing something, but it looks like the above method is only called with [[NSURL ...] absoluteString] and internally calls [NSURL URLWithString:URLString] straight away. What's the reason to jump between NSURL and NSString? Shouldn't be tried to keep the semantically correct data type as long/much/often as possible?

In the current situation one has to do the same NSURL<->NSString dance to use the request serializer.

If you agree, would you accept a PR?