Set 'fos_user_security_check' to support 'POST' requests only

Calling ``fos_user_security_check`` with ``GET`` leads to the error at https://github.com/FriendsOfSymfony/FOSUserBundle/blob/master/Controller/SecurityController.php#L70 . Restricting the page to ``POST`` will intercept the request on the router level, thereby preventing the error message.