Ensure the use of a correct model in the change handler

The change handler, used to update the changes in the view, uses in the current version the model passed in the change event. This situation can happen in situations like when using Backbone Associations. In case you are observing an relation, the changes in the related models are bubbled up to the parent model. This bubbling passes the _related_ model in the change event instead of the _parent_ model on which stickit is listening.

Of course you can dispute whether this is an issue in Backbone Associations, but it's anyway safer to refer to the correct model here and simply ignore the model passed in the event.
I think `m` is perfectly descriptive. Since it's not used anywhere in the function we really don't care about its name.