value binding on select doesn't respond to null when not using options binding

Discovered when trying to work around #472 if you don't use the options binding, it doesn't appear that knockout will drive the value binding when using null.  

Example case: http://jsfiddle.net/SfpVd/1/

Steps
1. go to the fiddle above
2. change the selection to 3
3. click the button
4. see how it does not reset