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**
);
    });
    describe('checkboxes', function() {
      itRenders('a checkbox that is checked with an onChange', async render => {
        const e = await render(
          <input type="checkbox" checked={true} onChange={() => {}} />,
        );
        expect(e.checked).toBe(true);
      });
      itRenders('a checkbox that is checked with readOnly', async render => {
        const e = await render(
          <input type="checkbox" checked={true} readOnly={true} />,
        );
        expect(e.checked).toBe(true);
      });
      itRenders(
        'a checkbox that is checked and no onChange/readOnly',
        async render => {
          
          
          const e = await render(<input type="checkbox" checked={true} />, 1);
          expect(e.checked).toBe(true);
        },
      );
      itRenders('a checkbox with defaultChecked', async render => {
        const e = await render(<input type="checkbox" defaultChecked={true} />);
        expect(e.checked).toBe(true);
        expect(e.getAttribute('defaultChecked')).toBe(null);
      });
      itRenders(