Summary: public
There was a race condition issue in RCTDownLoadTask whereby the request handler would sometimes call one of the delegate methods before setup was complete, causing an error to be logged because the request token had not been set, and causing te request to fail because the class was not yet set up.
This diff fixes that issue by adding an explicit `start` method to RCTDownloadTask, and changing the setup order to allow for the request to call back immediately without this being treated as an error.
Reviewed By: tadeuzagallo
Differential Revision: D2553628
fb-gh-sync-id: 5ca4e791574a632ccbf2e873e28ac88bffdf851d