NSProgresses should be created within context of currentProgress

There's currently an issue that prevents the progresses of upload and download tasks from being good citizens inside the NSProgress protocol.

First of all `AFURLSessionManagerTaskDelegate` creates the progress objects in its `-init` method and explicitly sets their parent to `nil`, so they'll never be added as children to `[NSProgress currentProgress]`.

The second issue is the fact that we're actually creating two progresses for upload and download. For a download task we're only updating the download progress though. The upload progress will block 50% of the pending changes in the parent progress though.

What's in this pull request:

- Tests for `AFURLSessionManager` that demonstrate the issues
- Updates to `AFURLSessionManager` to fix them