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**
if (ReactDOMFeatureFlags.useFiber) {
          
          if (
            render === serverRender ||
            render === clientRenderOnServerString ||
            render === streamRender
          ) {
            expect(e.childNodes.length).toBe(3);
            expectTextNode(e.childNodes[0], '<span>Text1&quot;</span>');
            expectTextNode(e.childNodes[2], '<span>Text2&quot;</span>');
          } else {
            expect(e.childNodes.length).toBe(2);
            expectTextNode(e.childNodes[0], '<span>Text1&quot;</span>');
            expectTextNode(e.childNodes[1], '<span>Text2&quot;</span>');
          }
        } else {
          
          
          expect(e.childNodes.length).toBe(6);
          expectTextNode(e.childNodes[0], '<span>Text1&quot;</span>');
          expectTextNode(e.childNodes[3], '<span>Text2&quot;</span>');
        }
      });
    });
    describe('components that throw errors', function() {
      itThrowsWhenRendering('a string component', async render => {
        const StringComponent = () => 'foo';
        await render(<StringComponent />, 1);
      });
      itThrowsWhenRendering('an undefined component', async render => {