React 16 alpha allows returning array in render function but server renderer expects children to always be react elements

```
TypeError: Cannot read property 'toLowerCase' of undefined
    at ReactDOMServerRenderer.renderDOM
```

source
```javascript
function renderDOM(element, context) {
    var tag = element.type.toLowerCase(); // element is array
```

Fiddle: https://jsfiddle.net/84v837e9/160/

**react@16.0.0-alpha.13**
var selectValue = this.currentSelectValue;
      var optionChildren = flattenOptionChildren(props.children);
      if (selectValue != null) {
        var value;
        if (props.value != null) {
          value = props.value + '';
        } else {
          value = optionChildren;
        }
        selected = false;
        if (Array.isArray(selectValue)) {
          
          for (var j = 0; j < selectValue.length; j++) {
            if ('' + selectValue[j] === value) {
              selected = true;
              break;
            }
          }
        } else {
          selected = '' + selectValue === value;
        }
        props = Object.assign(
          {
            selected: undefined,
            children: undefined,
          },
          props,
          {
            selected: selected,
            children: optionChildren,