Minimalist upgrade to AFNetworking 2.2.1

I took a very minimalist approach to doing the upgrade. The API stays exactly the same, I just accounted for three changes:

* Use `AFHTTPRequestOperationManager` because `AFHTTPClient` no longer exists.

* Change deserialization so that it will still use the old way if receiving `NSData` - this is mainly to keep the tests happy I think. The normal case should be that the `AFJSONResponseSerializer` does the work now.

* Make sure the test run again by explicitly sending results with application/json content type, because `AFJSONResponseSerializer` will reject them otherwise.