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

selectValue = ReactDOMSelect.getSelectValueContext(nativeParent);
    }
    
    
    var selected = null;
    if (selectValue != null) {
      selected = false;
      if (Array.isArray(selectValue)) {
        
        for (var i = 0; i < selectValue.length; i++) {
          if ('' + selectValue[i] === '' + props.value) {
            selected = true;
            break;
          }
        }
      } else {
        selected = ('' + selectValue === '' + props.value);
      }
    }
    inst._wrapperState = {selected: selected};
  },
  postMountWrapper: function(inst) {
    
    var props = inst._currentElement.props;
    if (props.value != null) {
      var node = ReactDOMComponentTree.getNodeFromInstance(inst);
      node.setAttribute('value', props.value);
    }
  },
  getNativeProps: function(inst, props) {