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**
expect(mountCount).toEqual(1);
      
      
      ReactDOM.unmountComponentAtNode(element);
      expect(element.innerHTML).toEqual('');
      ReactDOM.render(<TestComponent name="x" />, element);
      expect(mountCount).toEqual(2);
      expect(element.innerHTML).not.toEqual(lastMarkup);
      
      
      
      
      ReactDOM.unmountComponentAtNode(element);
      expect(element.innerHTML).toEqual('');
      ExecutionEnvironment.canUseDOM = false;
      lastMarkup = ReactDOMServer.renderToString(<TestComponent name="x" />);
      ExecutionEnvironment.canUseDOM = true;
      element.innerHTML = lastMarkup;
      var instance = ReactDOM.render(<TestComponent name="x" />, element);
      expect(mountCount).toEqual(3);
      var expectedMarkup = lastMarkup;
      if (ReactDOMFeatureFlags.useFiber) {
        var reactComments = /<!-- \/?react-text(: \d+)? -->/g;
        expectedMarkup = expectedMarkup.replace(reactComments, '');
      }
      expect(element.innerHTML).toBe(expectedMarkup);
      
      expect(numClicks).toEqual(0);
      ReactTestUtils.Simulate.click(ReactDOM.findDOMNode(instance.refs.span));
      expect(numClicks).toEqual(1);