Fix login_check route to prevent code 500 error on GET-request.

Since firewall's check_path handles only POST request the exception <code>throw new \RuntimeException('You must configure the check path to be handled by the firewall using form_login in your security firewall configuration.');</code> should be thrown only in case of POST request, and just 404 should be returned on GET. Otherwise we have 2 problems: 
- An error appears when all configured properly.
- On production environment each response with code 500 often configured to be logged and sent to admin, to fix the problem. So we have a route which is always lead to an error.