<select> fails to set a value when using <optgroup> (15.0 RC2)

[trevorsmith]

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

[jimfb]

Fixed in #6442