Applied fix for changes in google drive realtime api

This should resolve #5 

Checking for specific properties with `hasOwnProperty()` seems to be the method requiring the least amount of overhaul, but I'm not sure if it's robust enough.

An alternative would be to store a reference of the constructor during `MapReference` initialization, but that would require an overhaul of the `isFor` and `serialize` methods, since the contructor reference would be tied to a `MapReference` instance.