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 

} finally {
        ReactCurrentOwner.current = null;
      }
    } else {
      renderedComponent =
        this._renderValidatedComponentWithoutOwnerOrContext();
    }
    invariant(
      
      renderedComponent === null || renderedComponent === false ||
      React.isValidElement(renderedComponent),
      '%s.render(): A valid React element (or null) must be returned. You may have ' +
        'returned undefined, an array or some other invalid object.',
      this.getName() || 'ReactCompositeComponent'
    );
    return renderedComponent;
  },
  /**
   * Lazily allocates the refs object and stores `component` as `ref`.
   *
   * @param {string} ref Reference name.
   * @param {component} component Component to store as `ref`.
   * @final
   * @private
   */
  attachRef: function(ref, component) {
    var inst = this.getPublicInstance();
    invariant(inst != null, 'Stateless function components cannot have refs.');
    var publicComponentInstance = component.getPublicInstance();
    if (__DEV__) {