Add JSON to form format decoder

Converts JSON data to application/x-www-form-encoded compliant format.
Allo to use default Symfony's form framework widgets for JSON REST API.

* Unset `false` data to act like a checkbox
* Cast all non-string data to `string` like the default PHP behavior for $_ superglobals.

See for usage example: https://github.com/dunglas/DunglasTodoMVCBundle/blob/master/Controller/TodoController.php