@davidchambers excellent idea. I really like this. See aee8b86.

This produces an interesting additional effect. If the property is set to `undefined`, then it will use the `undefined` value. This is demonstrated in [the following test](https://github.com/jashkenas/underscore/pull/1515/files#diff-ce4d9e6321eda49ff09da21aa709e168R255). 

Overall, I am very happy with this revision, however I wanted to point this out.