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(<div>{'foo'}{'bar'}</div>);
        if (ReactDOMFeatureFlags.useFiber) {
          
          if (
            render === serverRender ||
            render === clientRenderOnServerString ||
            render === streamRender
          ) {
            
            expect(e.childNodes.length).toBe(3);
            expectTextNode(e.childNodes[0], 'foo');
            expectTextNode(e.childNodes[2], 'bar');
          } else {
            expect(e.childNodes.length).toBe(2);
            expectTextNode(e.childNodes[0], 'foo');
            expectTextNode(e.childNodes[1], 'bar');
          }
        } else {
          
          
          expect(e.childNodes.length).toBe(6);
          expectTextNode(e.childNodes[0], 'foo');
          expectTextNode(e.childNodes[3], 'bar');
        }
      });
    });
    describe('number children', function() {
      itRenders('a number as single child', async render => {
        const e = await render(<div>{3}</div>);
        expect(e.textContent).toBe('3');