remove mass assignment in model_change_tracking

This should fix https://github.com/chrismccord/sync/issues/124

The only mass assignment happening inside the gem which might cause this issue seems to be this one:

https://github.com/chrismccord/sync/blob/master/lib/sync/model_change_tracking.rb#L21

Essentially it creates a duplicate of the record to keep track of the applied changes. Assuming this is the cause, I refactored it and added some tests for the case of a model using attr_accessible.