added test demonstrating bug

Just the tests to demonstrate a problem documented here: https://groups.google.com/forum/#!msg/cujojs/8mmcCY1z6UY/Zn-Rq3z7BdMJ

Let me know if this is not quite the correct way to do this, I don't pretend to know the source at all, but hopefully this accurately demonstrates the problem and will allow a fix to be verified.