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 

'getDefaultProps was defined on %s, a plain JavaScript class. ' +
        'This is only supported for classes created using React.createClass. ' +
        'Use a static property to define defaultProps instead.',
        this.getName() || 'a component'
      );
      warning(
        !inst.propTypes,
        'propTypes was defined as an instance property on %s. Use a static ' +
        'property to define propTypes instead.',
        this.getName() || 'a component'
      );
      warning(
        !inst.contextTypes,
        'contextTypes was defined as an instance property on %s. Use a ' +
        'static property to define contextTypes instead.',
        this.getName() || 'a component'
      );
      warning(
        typeof inst.componentShouldUpdate !== 'function',
        '%s has a method called ' +
        'componentShouldUpdate(). Did you mean shouldComponentUpdate()? ' +
        'The name is phrased as a question because the function is ' +
        'expected to return a value.',
        (this.getName() || 'A component')
      );
      warning(
        typeof inst.componentDidUnmount !== 'function',
        '%s has a method called ' +
        'componentDidUnmount(). But there is no such lifecycle method. ' +
        'Did you mean componentWillUnmount()?',