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**
expect(this.wrapperRef.getTitle()).toBe('wrapper');
        expect(ReactDOM.findDOMNode(this.innerRef).className).toBe('inner');
        mounted = true;
      }
    }
    ReactTestUtils.renderIntoDocument(<Component />);
    expect(mounted).toBe(true);
  });
  it('should call refs at the correct time', () => {
    var log = [];
    class Inner extends React.Component {
      render() {
        log.push(`inner ${this.props.id} render`);
        return <div />;
      }
      componentDidMount() {
        log.push(`inner ${this.props.id} componentDidMount`);
      }
      componentDidUpdate() {
        log.push(`inner ${this.props.id} componentDidUpdate`);
      }
      componentWillUnmount() {
        log.push(`inner ${this.props.id} componentWillUnmount`);
      }
    }
    class Outer extends React.Component {