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**
expectTextNode(e.childNodes[4], ' ');
          } else {
            expect(e.childNodes.length).toBe(3);
            expectTextNode(e.childNodes[0], ' ');
            expectTextNode(e.childNodes[1], ' ');
            expectTextNode(e.childNodes[2], ' ');
          }
        } else {
          
          
          expect(e.childNodes.length).toBe(9);
          expectTextNode(e.childNodes[0], ' ');
          expectTextNode(e.childNodes[3], ' ');
          expectTextNode(e.childNodes[6], ' ');
        }
      });
      itRenders('a div with text sibling to a node', async render => {
        const e = await render(<div>Text<span>More Text</span></div>);
        let spanNode;
        if (ReactDOMFeatureFlags.useFiber) {
          
          
          expect(e.childNodes.length).toBe(2);
          spanNode = e.childNodes[1];
        } else {
          
          
          expect(e.childNodes.length).toBe(4);
          spanNode = e.childNodes[3];
        }