Ensures that the response is an actual Response.

When using the CSRF protection as a routing middleware, not applied in `kernel.php` it breaks because the controller/route could return a `View` instead of a `Response`. This is one potential fix.

Another approach would be to have the `view` function wrap the view in a `Response` to avoid this too.

e.g.

```php
class AuthController extends Controller {

	/**
	 * Create a new authentication controller instance.
	 *
	 * @param  Guard  $auth
	 * @return void
	 */
	public function __construct(Guard $auth)
	{
		$this->middleware('csrf');
	}

	/**
	 * Show the application registration form.
	 *
	 * @return Response
	 */
	public function getRegister()
	{
		return view('auth.login');
	}
```

The above returns a view to the middleware, where a response is expected.