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**
ReactDOM.unmountComponentAtNode(element);
      expect(element.innerHTML).toEqual('');
      
      
      element.innerHTML = lastMarkup;
      spyOn(console, 'error');
      instance = ReactDOM.render(<TestComponent name="y" />, element);
      expect(mountCount).toEqual(4);
      expectDev(console.error.calls.count()).toBe(1);
      expect(element.innerHTML.length > 0).toBe(true);
      expect(element.innerHTML).not.toEqual(lastMarkup);
      
      expect(numClicks).toEqual(1);
      ReactTestUtils.Simulate.click(ReactDOM.findDOMNode(instance.refs.span));
      expect(numClicks).toEqual(2);
    });
    it('should throw with silly args', () => {
      expect(
        ReactDOMServer.renderToString.bind(ReactDOMServer, 'not a component'),
      ).toThrowError('renderToString(): You must pass a valid ReactElement.');
    });
  });
  describe('renderToStaticMarkup', () => {
    it('should not put checksum and React ID on components', () => {
      class NestedComponent extends React.Component {
        render() {
          return <div>inner text</div>;
        }
      }
      class TestComponent extends React.Component {