Fix a realtime privacy leakage

[davidmz]

Fixed a realtime bug that caused data leakage via the 'post:update' and 'post:destroy' events.

The sample scenario was as follows:

• UserA subscribed to UserB, UserB subscribed to UserC, UserC is private;
• UserB likes post of UserC;
• UserC updates that post.

In this case, the UserA could receive the 'post:update' event with the full content of updated post (which is normally not available to him).