【Bugfix】Can not get user has authorized

the `JWTAuth::login` method, boolean value is returned, rather than the user instance. 

Reference:
1. https://github.com/dingo/api/blob/master/src/Auth/Authenticator.php#L79
2. https://github.com/tymondesigns/jwt-auth/blob/master/src/Tymon/JWTAuth/JWTAuth.php#L108
3. https://github.com/laravel/framework/blob/4.2/src/Illuminate/Auth/Guard.php#L473