Function extend does not properly expose static properties from base classes

[SylvainCorlay]

If a backbone model is an ES6 class with a statc property, and one use Model.extend to make a subclass (instead of the ES6 extends), the static properties of the base model are not added to the subclass.

[blikblum]

This is one of the issues with ES6 classes. See #3560 for discussion.

[SylvainCorlay]

@blikblum I have gone through this discussion earlier. I am not sure the specific issue that I am describing is covered in the discussion..

[oliverfoster]

@SylvainCorlay does my PR fix your issue? Static propery inheritance on the constructors?