Fixes chekedLink inconsistency (controlled inputs)

As stated on a [thread in ReactJS mailing list](https://groups.google.com/forum/#!topic/reactjs/rAOwWRpYI1M), there is an inconsistency when using `checkedLink` and `valueLink`. With the second are totally able to create a controlled component, whilst with the first, we aren't.

This PR fixes it by getting the value from the `LinkedValueUtils` just like how it is done for `valueLink`, making it consistent. 
This shouldn't actually be needed I think. It should be safe to just put `null` instead of `this.handleChange` for ReactLink.