Check for selector, not class when retrieving request object from AFURLSessionManager

The class of the object associated with a notification sent from AFURLSessionManager may not be NSURLSessionTask e.g. [[notification object] class] => __NSCFLocalDataTask.
Check for respondsToSelector: in order to retrieve the request instead.
Merge pull request #2 from arlophoenix/master

Check for selector, not class when retrieving request object from AFURLSessionManager