used on boolean field change callback not triggered when update to false

[Jcbobo]

if set up the mixins on a boolean property only the change from false to true is detected (and callback is called) but transaction from true to false was not detected, my pr solve the issue

[mannyhenri]

@mrfelton fyi I still get notifications on several repos from fullcube. I unsubscribe whenever I see one but wonder why I still get those.

[Jcbobo]

close #25