isValidatable Bulletproofing

Inside of the isValidatable function I added a line to return false if the "o" argument is undefined. It was in a few of my scenarios and adding this line fixed a bug for me, I realize some other issue existed, but it seemed an appropriate bulletproofing line for a library. If you're not opposed to this I'd appreciate having it in future versions. I'm thinking it's harmless and will add value.