Additional "Unify Manager._update behaviour" cleanup

Make _update always return an instance of self.resource_class.

This is in preparation for fixing bug 1145768 by reverting the API
changes made in the original "Unify Manager._update behaviour" change.

This is to avoid needing to revert some of the cleanup that was made
in that change.

Change-Id: I842bda40a0dc168689a7dd8b88433c3cebef5839
Merge "Additional "Unify Manager._update behaviour" cleanup"