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**
}
      });
      itRenders('a div with a child', async render => {
        const e = await render(<div id="parent"><div id="child" /></div>);
        expect(e.id).toBe('parent');
        expect(e.childNodes.length).toBe(1);
        expect(e.childNodes[0].id).toBe('child');
        expect(e.childNodes[0].childNodes.length).toBe(0);
      });
      itRenders('a div with multiple children', async render => {
        const e = await render(
          <div id="parent"><div id="child1" /><div id="child2" /></div>,
        );
        expect(e.id).toBe('parent');
        expect(e.childNodes.length).toBe(2);
        expect(e.childNodes[0].id).toBe('child1');
        expect(e.childNodes[0].childNodes.length).toBe(0);
        expect(e.childNodes[1].id).toBe('child2');
        expect(e.childNodes[1].childNodes.length).toBe(0);
      });
      itRenders(
        'a div with multiple children separated by whitespace',
        async render => {
          const e = await render(
            <div id="parent"><div id="child1" /> <div id="child2" /></div>,
          );
          expect(e.id).toBe('parent');
          let child1, child2, textNode;
          if (ReactDOMFeatureFlags.useFiber) {