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 

log.push('Boundary componentDidMount');
      }
      componentWillUnmount() {
        log.push('Boundary componentWillUnmount');
      }
      onClick() {
        
      }
      unstable_handleError() {
        this.setState({error: true});
      }
    }
    var renderer = ReactTestRenderer.create(<Boundary />);
    expect(renderer.toJSON()).toEqual({
      type: 'div',
      props: {},
      children: ['Happy Birthday!'],
    });
    expect(log).toEqual([
      'Boundary render',
      'Angry render',
      'Boundary render',
      'Boundary componentDidMount',
    ]);
  });
});