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 maskedContext = this._maskContext(context);
    if (__DEV__) {
      var Component = this._currentElement.type;
      if (Component.contextTypes) {
        this._checkContextTypes(
          Component.contextTypes,
          maskedContext,
          'context'
        );
      }
    }
    return maskedContext;
  },
  /**
   * @param {object} currentContext
   * @return {object}
   * @private
   */
  _processChildContext: function(currentContext) {
    var Component = this._currentElement.type;
    var inst = this._instance;
    var childContext;
    if (inst.getChildContext) {
      if (__DEV__) {
        ReactInstrumentation.debugTool.onBeginProcessingChildContext();
        try {
          childContext = inst.getChildContext();
        } finally {
          ReactInstrumentation.debugTool.onEndProcessingChildContext();
        }