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**
componentWillMount() {
        this.forceUpdate();
        setTimeout(() => {
          this.forceUpdate();
        });
      }
      render() {
        return <div onClick={() => {}} />;
      }
    }
    spyOn(console, 'error');
    ReactDOMServer.renderToString(<Baz />);
    jest.runOnlyPendingTimers();
    expectDev(console.error.calls.count()).toBe(1);
    expectDev(console.error.calls.mostRecent().args[0]).toBe(
      'Warning: forceUpdate(...): Can only update a mounting component. ' +
        'This usually means you called forceUpdate() outside componentWillMount() on the server. ' +
        'This is a no-op.\n\nPlease check the code for the Baz component.',
    );
    var markup = ReactDOMServer.renderToStaticMarkup(<Baz />);
    expect(markup).toBe('<div></div>');
  });
  it('should warn when children are mutated during render', () => {
    spyOn(console, 'error');
    function Wrapper(props) {