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 delegates progress property.
When using -[NSProgress becomeCurrentWithPendingUnitCount:] two NSProgress children are added, as two are created in the current parent's context.