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(e.childNodes.length).toBe(3);
          expectTextNode(e.childNodes[0], 'foo');
        }
      });
      itRenders('only null and false children as blank', async render => {
        const e = await render(<div>{false}{null}{null}{false}</div>);
        expect(e.childNodes.length).toBe(0);
      });
    });
    describe('elements with implicit namespaces', function() {
      itRenders('an svg element', async render => {
        const e = await render(<svg />);
        expect(e.childNodes.length).toBe(0);
        expect(e.tagName).toBe('svg');
        expect(e.namespaceURI).toBe('http:www.w3.org/2000/svg');
      });
      itRenders('svg element with an xlink', async render => {
        let e = await render(
          <svg><image xlinkHref="http:i.imgur.com/w7GCRPb.png" /></svg>,
        );
        e = e.firstChild;
        expect(e.childNodes.length).toBe(0);
        expect(e.tagName).toBe('image');
        expect(e.namespaceURI).toBe('http:www.w3.org/2000/svg');
        expect(e.getAttributeNS('http:www.w3.org/1999/xlink', 'href')).toBe(
          'http:i.imgur.com/w7GCRPb.png',
        );
      });
      itRenders('a math element', async render => {