Add test for custom unmapping via ko.viewmodel.toModel

[aknuds1]

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 I opened previously regarding this problem.

[coderenaissance]

responded on #40, thanks.