workaround for knockout JS creation problem

Dear Oscar, 

I took the liberty to add a new function to the view model: toJSONByPrototype when the method extendWithPage is executed.

This allows one to generate the JS by a model prototype. When I download a data model from server side and create the viewmodel for knockout, I can use this as a "prototype" for JS creation. 

This will prevent the knockout to go and touch properties which do not belong to the model.

Considering, that your library must be functional on almost any knockout lib, i post this change here and not to the knockout lib.