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**
log.push('outer componentDidMount');
      }
      componentDidUpdate() {
        log.push('outer componentDidUpdate');
      }
      componentWillUnmount() {
        log.push('outer componentWillUnmount');
      }
    }
    
    var el = document.createElement('div');
    log.push('start mount');
    ReactDOM.render(<Outer />, el);
    log.push('start update');
    ReactDOM.render(<Outer />, el);
    log.push('start unmount');
    ReactDOM.unmountComponentAtNode(el);
    
    expect(log).toEqual([
      'start mount',
      'inner 1 render',
      'inner 2 render',
      'inner 1 componentDidMount',
      'ref 1 got instance 1',
      'inner 2 componentDidMount',
      'ref 2 got instance 2',
      'outer componentDidMount',
      'start update',