[5.4] Add json alias for decodeResponseJson method

[jerguslejko]

This PR adds json() method to Illuminate\Foundation\Testing\TestResponse.

This method serves as an alias for decodeResponseJson() method.

It allows for nicer and much more intuitive API:

function it_gets_an_order_by_id()
{
    $order = $this->get('/orders/1')->decodeResponseJson();

    // vs

    $order = $this->get('/orders/1')->json();
}

🌵