[TIMOB-18780] iOS: Add ability for Background Fetch tasks to support JSON responses

Feature Request

Add ability to get a JSON response for Background Fetch task. When you try to use same url using url session module, response doesn't contain text instead of data as empty JS object of "downloadcompleted" event.

[INFO]  e{"taskIdentifier":1,"data":{},"bubbles":true,"type":"downloadcompleted","source":{},"cancelBubble":false}
[INFO]  sessioncompleted {"errorCode":0,"taskIdentifier":1,"message":"","success":true,"bubbles":true,"type":"sessioncompleted","source":{},"cancelBubble":false}
[INFO]  Downloads completed successfully.
[INFO]  Download task :1 completed, success ? : true

Appcelerator Docs

http://docs.appcelerator.com/titanium/3.0/#!/guide/iOS_Background_Services

Sample Code

https://gist.github.com/egomez99/2d968326454e0dd8401c

Additional info

downloadcompleted event currently only supports Titanium.Blob data type for a download nsurlsession task. http://docs.appcelerator.com/platform/latest/#!/api/Titanium.App.iOS-event-downloadcompleted

Apple Docs

https://developer.apple.com/library/ios/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BackgroundExecution/BackgroundExecution.html https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/URLLoadingSystem/URLLoadingSystem.html#//apple_ref/doc/uid/10000165i