Notifications content type enabled not using strict checking on in_array

When a content type has subscriptions turned off, the check in notifications_content_type_enabled() incorrectly returns true. This is caused by the fact that the in_array call isn't using strict checking (despite the fact that in the function's comments it says "PHP Note: We need to use strict checking for in_array(), http://es.php.net/manual/en/function.in-array.php#91911")

Pretty simple patch but it fixes another important part of notifications.