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(5);
          expectTextNode(e.childNodes[0], '');
          expectTextNode(e.childNodes[2], 'foo');
        }
      });
      itRenders('a trailing blank child with a text sibling', async render => {
        const e = await render(<div>foo{''}</div>);
        if (ReactDOMFeatureFlags.useFiber) {
          
          if (render === serverRender || render === streamRender) {
            expect(e.childNodes.length).toBe(1);
            expectTextNode(e.childNodes[0], 'foo');
          } else {
            expect(e.childNodes.length).toBe(2);
            expectTextNode(e.childNodes[0], 'foo');
            expectTextNode(e.childNodes[1], '');
          }
        } else {
          
          
          
          expect(e.childNodes.length).toBe(5);
          expectTextNode(e.childNodes[0], 'foo');
          expectTextNode(e.childNodes[3], '');
        }
      });
      itRenders('an element with two text children', async render => {