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

ReactDOM.unmountComponentAtNode(container);
    }
    var container = document.createElement('div');
    var stub =
      <select value="giraffe" onChange={changeView}>
        <option value="monkey">A monkey!</option>
        <option value="giraffe">A giraffe!</option>
        <option value="gorilla">A gorilla!</option>
      </select>;
    stub = ReactDOM.render(stub, container);
    var node = ReactDOM.findDOMNode(stub);
    expect(() => ReactTestUtils.Simulate.change(node)).not.toThrow(
      "Cannot set property 'pendingUpdate' of null"
    );
  });
});