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 

*/
  getName: function() {
    var type = this._currentElement.type;
    var constructor = this._instance && this._instance.constructor;
    return (
      type.displayName || (constructor && constructor.displayName) ||
      type.name || (constructor && constructor.name) ||
      null
    );
  },
  /**
   * Get the publicly accessible representation of this component - i.e. what
   * is exposed by refs and returned by render. Can be null for stateless
   * components.
   *
   * @return {ReactComponent} the public component instance.
   * @internal
   */
  getPublicInstance: function() {
    var inst = this._instance;
    if (this._compositeType === CompositeTypes.StatelessFunctional) {
      return null;
    }
    return inst;
  },
  
  _instantiateReactComponent: null,
};
module.exports = ReactCompositeComponent;