PINRemoteImage: Add KVO to NSURLSessionTask may cause crash.

The crashes are here: http://crashes.to/s/45830cf8ec3

I found the same issue discussed about AFNetworking. https://github.com/AFNetworking/AFNetworking/issues/1477

I searched all the code in my project that the category NSURLSessionTask+Timing with the observer on the property of NSURLSessionTask may cause the crash.