Use json/application Content-Type headers for payload in ParseOrbiter

On first setup, I got the following error from Parse:
`{"code":107,"error":"This endpoint only supports Content-Type: application/json requests, not application/x-www-form-urlencoded."}`. Request headers were obviously wrong `"Content-Type" = "application/x-www-form-urlencoded; charset=utf-8"`. Seems like Parse requires json/application Content-Type for payload.

Now it works fine. Love this lightweight wrapper around push APIs (versus Parse's 5mb+ giant kit)!