Anonynous notifications are received even for non-watched posts

If an anonymous user subscribes to 1 post, he will also receive notifications for any other updated post (not subscribed).

Here is how to reproduce the issue:
- Let's say you have 3 'story' posts A, B and C.
- The anonymous user registers to Post A updates (comments and post update).
- An editor updates post C
=> The anonymous user receives a notification corresponding to post C update! (problem)
=> The link to un-subscribe to this post notifications (post C) has no effect at all (another post C update will generate another notification)
- An editor updates post A
=> The anonymous user receives a notification corresponding to post A update. (this is correct)
=> Using the embedded link to 'un-subscribe to post A notification' works, and will also fix the un-expected notification on post C updates.