React 0.14.8 correct sets the defaultValue for `<select>` when using `<optgroup>`:
https://jsfiddle.net/6v8cnowo/

However, 15.0 RC2 defaults to the first `<option>`:
https://jsfiddle.net/p2ctcgwj/1/

I'm not very familiar with the React codebase, but I suspect it has something to do with a change made in 1dca72e, specifically this line: https://github.com/facebook/react/blame/master/src/renderers/dom/client/wrappers/ReactDOMOption.js#L35

* Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule ReactDOMOption
 */
'use strict';
var ReactChildren = require('ReactChildren');
var ReactDOMComponentTree = require('ReactDOMComponentTree');
var ReactDOMSelect = require('ReactDOMSelect');
var warning = require('warning');
/**
 * Implements an <option> native component that warns when `selected` is set.
 */
var ReactDOMOption = {
  mountWrapper: function(inst, props, nativeParent) {
    
    if (__DEV__) {
      warning(
        props.selected == null,
        'Use the `defaultValue` or `value` props on <select> instead of ' +
        'setting `selected` on <option>.'
      );
    }
    
    var selectValue = null;
    if (nativeParent != null && nativeParent._tag === 'select') {