[6.x] Add "unauthenticated" method in auth middleware

[TitasGailius]

It would be nice to have an easy way to customise the "unauthenticated" behaviour in Authenticate middleware.

For example, I'd like to display a 404 page when the user is unauthenticated.

Of course, one solution might be to catch AuthenticationException in the exception handler and adjust the behaviour accordingly but I find it much cleaner to do it in the middleware.

With this PR I could simply override the unauthenticated method in the Authenticate middleware that ships with Laravel.

class Authenticate extends Middleware
{
    /**
     * Handle unauthenticated user.
     *
     * @return void
     *
     * @throws \Symfony\Component\HttpKernel\Exception\NotFoundHttpException
     */
    protected function unauthenticated()
    {
        throw new NotFoundHttpException;
    }
}

[palokankare]

Typo in method parameter $gaurds

[m1guelpf]

Suggested change

	protected function unauthenticated($request, array $gaurds)
	protected function unauthenticated($request, array $guards)

[TitasGailius]

wow sorry, I don't know how I missed that. thanks!