Remove use of instanceof

instanceof does not work correctly for cross frame code.  Removed it's use in mapJsObjectGraph to fix ko.toJS on cross-frame objects.

See here for example of problem caused:
http://stackoverflow.com/questions/24890168/ko-tojs-converting-array-to-object/

And here for a bug report for using instanceof in general:
https://github.com/knockout/knockout/issues/1307