Make sure inverse_of is visible on the has_many callbacks

### Problem
When inserting a children object and the children element has a callback that call its parent, even having the `inverse_of`, it does an extra query to populate the parent object.

### Solution
When insert the record, we need to load the inverse instance before calling save, so the callbacks will have access to it.

review @rafaelfranca @carlosantoniodasilva

Also I would :green_heart:  to have that backported to rails 3.2 if possible.