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**
constructor(element, makeStaticMarkup) {
    
    
    super({});
    this.partialRenderer = new ReactPartialRenderer(element, makeStaticMarkup);
  }
  _read(size) {
    try {
      this.push(this.partialRenderer.read(size));
    } catch (err) {
      this.emit('error', err);
    }
  }
}
/**
 * Render a ReactElement to its initial HTML. This should only be used on the
 * server.
 * See https://facebook.github.io/react/docs/react-dom-stream.html#rendertostream
 */
function renderToStream(element) {