If you attach a `ref` to an element using a string (`ref='foo'`) and use `ReactTestRenderer` you get the `"Stateless function components cannot be given refs"` warning

See https://github.com/facebook/react/issues/7371#issuecomment-237816352

This is because `getPublicInstance` returns `null` and `attachRef` requires that the instance be non-null in the warning invariant.

I think we can either return a simple instance instead of `null` or use some additional check in `attachRef` to see if we're dealing with `ReactTestRenderer`. The first option seems reasonable, given the comment in the current code: 

``` js
ReactTestComponent.prototype.getPublicInstance = function() {
  // I can't say this makes a ton of sense but it seems better than throwing.
  // Maybe we'll revise later if someone has a good use case.
  return null;
};
```

cc @spicyj 

render() {
        log.push('Angry render');
        throw new Error('Please, do not render me.');
      }
      componentDidMount() {
        log.push('Angry componentDidMount');
      }
      componentWillUnmount() {
        log.push('Angry componentWillUnmount');
      }
    }
    class Boundary extends React.Component {
      constructor(props) {
        super(props);
        this.state = {error: false};
      }
      render() {
        log.push('Boundary render');
        if (!this.state.error) {
          return (
            <div><button onClick={this.onClick}>ClickMe</button><Angry /></div>
          );
        } else {
          return <div>Happy Birthday!</div>;
        }
      }
      componentDidMount() {