Fix for NSProgress bug where two child NSProgress instances are added to a parent NSProgress.
    AFURLSessionManagerTaskDelegate creates a NSProgress in its initializer. However, -[AFURLSessionManager addDelegateForUploadTask:progress:completionHandler:] assigns a new NSProgress to the
    When using -[NSProgress becomeCurrentWithPendingUnitCount:] two NSProgress children are added, as two are created in the current parent's context.