Property changed events for mix:lastModified should only be fired if the underlying node actually changes

ModeShape normally only fires PROPERTY_CHANGED_EVENT s for those properties which have changed values during the course of a session.save. However, if a property is set to the same value as its previous existing value(i.e. a no-op), these events are not fired.

This is not the case atm. for mix:lastModified nodes, for which both the jcr:lastModified and jcr:lastModifiedBy are set regardless of the actual values on each session.save. We should make sure that the these properties are set (and therefore events fired) only if there are actual changes on a particular node, compared to its previous state.

The current behavior is not necessarily a bug (as per JCR #3.7.11.8), but we want to be consistent with respect to the property changed events.