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 

nextProps,
    nextState,
    nextContext,
    transaction,
    unmaskedContext
  ) {
    var inst = this._instance;
    var hasComponentDidUpdate = Boolean(inst.componentDidUpdate);
    var prevProps;
    var prevState;
    var prevContext;
    if (hasComponentDidUpdate) {
      prevProps = inst.props;
      prevState = inst.state;
      prevContext = inst.context;
    }
    if (inst.componentWillUpdate) {
      if (__DEV__) {
        measureLifeCyclePerf(
          () => inst.componentWillUpdate(nextProps, nextState, nextContext),
          this._debugID,
          'componentWillUpdate'
        );
      } else {
        inst.componentWillUpdate(nextProps, nextState, nextContext);
      }
    }
    this._currentElement = nextElement;
    this._context = unmaskedContext;
    inst.props = nextProps;