More careful handling of completed NSData

When the connection completes successfully, then it is known that the
data, self.mutableData, is complete. Representing completeness of the
data specificly, self.finishedData, seems to make a more accurate
reflection than isFinished state as logical criteria for purpose of
responseData method behavior. isFinished is the signal NSOperationQueue
uses to dequeue the operation - an orthogonal concern to data
completeness.