Minor bug when formatting undefined fields

Hey there, thanks for writing this lib.  I was using it and seeing a javascript error on jquery.github.js line 86: "Uncaught TypeError: Cannot read property 'length' of null".  So I think it didn't like checking the length of a thing that wasn't defined.  I changed it to "=== "string"", since that seemed to work better.  Just an idea, thanks again.