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 

},
  /**
   * Filters the context object to only contain keys specified in
   * `contextTypes`
   *
   * @param {object} context
   * @return {?object}
   * @private
   */
  _maskContext: function(context) {
    var Component = this._currentElement.type;
    var contextTypes = Component.contextTypes;
    if (!contextTypes) {
      return emptyObject;
    }
    var maskedContext = {};
    for (var contextName in contextTypes) {
      maskedContext[contextName] = context[contextName];
    }
    return maskedContext;
  },
  /**
   * Filters the context object to only contain keys specified in
   * `contextTypes`, and asserts that they are valid.
   *
   * @param {object} context
   * @return {?object}
   * @private
   */
  _processContext: function(context) {