JsonResponse should implement JsonSerializable

[Rican7]

The Paulus\Response\JsonResponse should implement the newly available (since PHP 5.4) JsonSerializable class.

I'm thinking that instead of encoding the body every time new data is set on the JsonResponse class to instead have some sort of preparation before sending based on whether the class implements the JsonSerializable interface or not. Maybe some sort of middleware layer? Not sure.