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 

state = {x: 3};
      render() {
        renders++;
        return (
          <div className="purple">
            {this.state.x}
            <Child />
            <Null />
          </div>
        );
      }
      componentDidMount() {
        this.setState({x: 7});
      }
    }
    var Child = () => (renders++, <moo />);
    var Null = () => (renders++, null);
    var renderer = ReactTestRenderer.create(<Component />);
    expect(renderer.toJSON()).toEqual({
      type: 'div',
      props: { className: 'purple' },
      children: [
        7,
        { type: 'moo', props: {}, children: null },
      ],
    });
    expect(renders).toBe(6);
  });
  it('exposes the instance', function() {
    class Mouse extends React.Component {