Laravel 5 unit testing. Unable to set JSON of Request object

How on earth do you get Laravel 5.0 to accept a JSON encoded string into it's request object? Because my REST api is returning 500 errors, and upon closer inspection, the request object has an empty json property...?

My array:

    private $test_1_create_user = array(
        "name" => "Mr T Est",
        "email" => "mrtest@somedomain.com",
        "password" => "testing1234"
    );

My test method:

    /**
    * Attempts to Create a single user with no permissions
    */
    public function testCreateUser(){
        /** Obtain instance of Request object */
        $req = $this->app->request->instance();
        /** Set the JSON packet */
        $req->json(json_encode($this->test_1_create_user));
        /** Run the test */
        $response = $this->call('POST', '/api/v1/user');
        /** Read the response */    
        $this->assertResponseOk();
    }

And a var_dump of $req (slimmed down a bit):

C:\wamp\www\nps>php phpunit.phar
PHPUnit 4.6.2 by Sebastian Bergmann and contributors.

Configuration read from C:\wamp\www\nps\phpunit.xml

class Illuminate\Http\Request#34 (25) {
  protected $json =>
  class Symfony\Component\HttpFoundation\ParameterBag#261 (1) {
    protected $parameters =>
    array(0) {
    }
  }
  protected $sessionStore =>
  NULL
  protected $userResolver =>
  NULL
  protected $routeResolver =>
  NULL
  public $attributes =>
  class Symfony\Component\HttpFoundation\ParameterBag#41 (1) {
    protected $parameters =>
    array(0) {
    }
  }
  public $request =>
  class Symfony\Component\HttpFoundation\ParameterBag#43 (1) {
    protected $parameters =>
    array(0) {
   }
 }

It took me quite a while to figure out how to access the request object from within a unit test. Anyone have any ideas as to why the $req->json is always empty? :( Cheers!