Redirecting to the login page after fail request validation in laravel 5.1

I am creating Rest Full Api for mobile application, I am validating request it redirects me to the login page with errors.

Here is my ApiController (I have created for all api):

use App\User as UserModel;
use App\Fb_friend as FbFriendsModel;
use App\Http\Requests\UserRequest;

class ApiController extends Controller
{

    /**
     * Create a new movie model instance.
     *
     * @return void
     */
    public function __construct(UserModel $user, FbFriendsModel $fb_friends){
        $this->user = $user;
        $this->fb_friends = $fb_friends;
    }
    public function createUser (UserRequest $request) {
      // some code here
    }

Route:

Route::post('createUser', ['as' => 'createUser', 'uses' => 'ApiController@createUser']);

UserRequest.php:

public function rules()
    {
        return [
            'fb_id' => 'required|unique:users',
            'username' => 'required|unique:users',
            'email' => 'required|unique:users',
            'image' => 'required',
            'device_id' => 'required',
            'status' => 'required',
        ];
    }

I have override a function Request.php for error formatting:

abstract class Request extends FormRequest
{
    protected function formatErrors(Validator $validator)
    {
        return [$validator->messages()->toJson()];
    }
}

When I try to call service via postman, it returns me error in json format but it also print the login page, I m not getting why?

via Chebli Mohamed