Pass signal to ModelBase's update

Paging @kumar303

Can you verify this is a good change and I'm not changing something you intentionally did?

What I found was that when I wanted no signals to be fired and I was calling `update` on a model object (e.g. `webapp.update(status=PUBLIC)`), the object's `update` was the one defined in `OnChangeMixin`, which then calls it's parent class' `update` which is the one defined in `ModelBase`. But when it calls `ModelBase`s `update` it has already stripped the `_signal` kwarg and the signal ends up getting called anyway.

This change passes it up to the parent class.