Workaround radar 14783825

Testing for `[[notification object] isKindOfClass:[NSURLSessionTask class]]` should be enough but NSURLSessionTask subclasses lie about their class membership. See http://www.openradar.me/14783825

This commit can be hopefully reverted if rdar://problem/14783825 is fixed before iOS 7 goes public.
Simply test for `respondsToSelector:` instead of `isKindOfClass:` chain
Merge pull request #1242 from 0xced/workaround-radar-14783825

[2.0] Workaround radar 14783825