Fix for quick canceling request operation

I noticed that testThatCancellationOfRequestOperationInvokesFailureCompletionBlock fails a lot when I run tests locally. It looks like if you cancel the operation quick enough operationDidStart isn't finished and the failure completion block doesn't get called with an error. I added code to operationDidStart to set the error the same way as if it were to happen when the request has actually started.