Fix json_decode in JsonResponse for PHP 5.3

Urgent bugfix, problem introduced in https://github.com/laravel/framework/commit/417f539803ce96eeab7b420d8b03f04959a603e9

PHP 5.3 does not allow 4 parameters for json_decode, and results in a fatal error.

I left the options flag in for the json_encode bit but feel that this might come off as inconsistent behaviour - allowing options on encoding but not decoding.