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

/**