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 

constructor() {
        super();
        this.state = {mouse: 'mouse'};
      }
      handleMoose() {
        this.setState({mouse: 'moose'});
      }
      render() {
        return <div>{this.state.mouse}</div>;
      }
    }
    var renderer = ReactTestRenderer.create(<Mouse />);
    expect(renderer.toJSON()).toEqual({
      type: 'div',
      props: {},
      children: ['mouse'],
    });