Retrieve JSON as collection instead of ParameterBag.

Previously the `Input::json()` was retrieved as a `Symfony ParameterBag` which meant that it was missing a number of retrieval methods that the Request uses for POST/GET data. Namely `only()` and `except()`.

I have replaced this with an `Illuminate\Support\Collection` to grant these new retrieval methods. All tests are passing, and the most useful methods of the parameterbag are present on the collection. It should offer a seamless move, unless someone is doing something very fiddly with it. :)