Classes dont get added to child with ref.

Migrating from v1 to v2 and nothing is working

```
this.ref = React.createRef();
....

<CSSTransition nodeRef={this.ref} in={true} classNames="my-class" timeout={1000}>
     <span ref={this.ref}>Hello???</span>
 </CSSTransition>
```

I can see that no classes get applied to the `span`. Surly this basic functionality must be working.

