Add support for 'undefined' for better binding support, especially in di...

...rectives with watchers

Great handy library -- I am submitting a patch for what I imagine to be a common occurrence. If your object is the result of an AJAX call, the library spews some console errors when it is initially undefined. Then proceeds to work as normal. See this fiddle:
http://jsfiddle.net/brianmgray/sf0famvs/

This patch just adds a check for undefined and returns undefined. I added a test as well.
`return;` is good enough :>