Fix bug when using custom Auth driver

When attempting to use a custom Auth driver, an error is thrown due to an incorrect check in the `callCustomCreator` method that's been overridden in the customised `AuthManager` class.

This is because `parent::callCustomCreator($driver);` will return an instance of `Illuminate\Auth\Guard`, and consequently the `instanceof` check will fail. In order to ensure that the correct `Guard` class is created (and that the `instanceof` check passes), I've had to put in a call directly to the `Illuminate\Support\Manager` class. This is a bit dirty but is unfortunately the only workaround with the way the code is structured.