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() {
          const staticContent = ReactDOMServer.renderToStaticMarkup(
            <div>
              <img src="foo-bar.jpg" />
            </div>,
          );
          return <div dangerouslySetInnerHTML={{__html: staticContent}} />;
        }
      }
      class Component extends React.Component {
        componentWillMount() {
          this.setState({text: 'hello, world'});
        }
        render() {
          return <div>{this.state.text}</div>;
        }
      }
      expect(
        ReactDOMServer.renderToString.bind(
          ReactDOMServer,
          <div>
            <StaticComponent />
            <Component />
          </div>,
        ),
      ).not.toThrow();
    });
  });
  it('warns with a no-op when an async setState is triggered', () => {
    class Foo extends React.Component {