Add back test for user rejected value

Not sure how this got lost. Asserts that https://github.com/Polymer/NodeBind/blob/master/src/NodeBind.js#L186 is required (otherwise the element wouldn't find out that the value was put back to its initial value, because from it's observable's POV the value would not have changed).

R=arv
BUG=

Review URL: https://codereview.appspot.com/73240044