Add test for custom unmapping via ko.viewmodel.toModel

I've added a test for the scenario where you piggyback on ko.viewmodel.fromModel in a custom mapping function and ko.viewmodel.toModel in a custom unmapping function. This scenario currently leads to infinite recursion in the unmapping function. See the [issue](https://github.com/coderenaissance/knockout.viewmodel/issues/40) I opened previously regarding this problem.