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 

}
    var nextState = this._processPendingState(nextProps, nextContext);
    var shouldUpdate = true;
    if (!this._pendingForceUpdate) {
      if (inst.shouldComponentUpdate) {
        if (__DEV__) {
          shouldUpdate = measureLifeCyclePerf(
            () => inst.shouldComponentUpdate(nextProps, nextState, nextContext),
            this._debugID,
            'shouldComponentUpdate'
          );
        } else {
          shouldUpdate = inst.shouldComponentUpdate(nextProps, nextState, nextContext);
        }
      } else {
        if (this._compositeType === CompositeTypes.PureClass) {
          shouldUpdate =
            !shallowEqual(prevProps, nextProps) ||
            !shallowEqual(inst.state, nextState);
        }
      }
    }
    if (__DEV__) {
      warning(
        shouldUpdate !== undefined,
        '%s.shouldComponentUpdate(): Returned undefined instead of a ' +
        'boolean value. Make sure to return true or false.',
        this.getName() || 'ReactCompositeComponent'
      );
    }