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. 
```
    var RadioInput = React.createClass({
      render: function () {
        return (
          <input
            type="radio"
            checkedLink={new ReactLink(false, null)} />
        );
      }
    });
```

Should be enough I think.