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**
render() {
            return <div ref="foo" />;
          }
        }
        const e = await render(<RefComponent />);
        expect(e.getAttribute('ref')).toBe(null);
      });
      itRenders('no children attribute', async render => {
        const e = await render(React.createElement('div', {}, 'foo'));
        expect(e.getAttribute('children')).toBe(null);
      });
      itRenders('no key attribute', async render => {
        const e = await render(<div key="foo" />);
        expect(e.getAttribute('key')).toBe(null);
      });
      itRenders('no dangerouslySetInnerHTML attribute', async render => {
        const e = await render(
          <div dangerouslySetInnerHTML={{__html: '<foo />'}} />,
        );
        expect(e.getAttribute('dangerouslySetInnerHTML')).toBe(null);
      });
    });