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**
const e = await render(<math />);
        expect(e.childNodes.length).toBe(0);
        expect(e.tagName).toBe('math');
        expect(e.namespaceURI).toBe('http:www.w3.org/1998/Math/MathML');
      });
    });
    
    
    itRenders('an img', async render => {
      const e = await render(<img />);
      expect(e.childNodes.length).toBe(0);
      expect(e.nextSibling).toBe(null);
      expect(e.tagName).toBe('IMG');
    });
    itRenders('a button', async render => {
      const e = await render(<button />);
      expect(e.childNodes.length).toBe(0);
      expect(e.nextSibling).toBe(null);
      expect(e.tagName).toBe('BUTTON');
    });
    itRenders('a div with dangerouslySetInnerHTML', async render => {
      const e = await render(
        <div dangerouslySetInnerHTML={{__html: "<span id='child'/>"}} />,
      );
      expect(e.childNodes.length).toBe(1);
      expect(e.firstChild.tagName).toBe('SPAN');
      expect(e.firstChild.getAttribute('id')).toBe('child');
      expect(e.firstChild.childNodes.length).toBe(0);
    });
    describe('newline-eating elements', function() {