Bugfix: 'filePath' is nil if the manager didFinishDownloadingBlock returns URL

This PR fixes an issue where the filePath variable is nil for __downloadTaskWithRequest:progress:destination:completionHandler:__ completion handler if the local URL for the task is decided by the manager block.

This includes PR for #2401 , since I wanted to make sure the unit tests were green, and ideally #2401 is merged first.