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**
'a checkbox checked overriding defaultChecked',
        async render => {
          const e = await render(
            <input
              type="checkbox"
              checked={true}
              defaultChecked={false}
              readOnly={true}
            />,
            1,
          );
          expect(e.checked).toBe(true);
          expect(e.getAttribute('defaultChecked')).toBe(null);
        },
      );
      itRenders(
        'a checkbox checked overriding defaultChecked no matter the prop order',
        async render => {
          const e = await render(
            <input
              type="checkbox"
              defaultChecked={false}
              checked={true}
              readOnly={true}
            />,
            1,
          );
          expect(e.checked).toBe(true);
          expect(e.getAttribute('defaultChecked')).toBe(null);
        },