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**
constructor() {
          super();
        }
        render() {
          return <div>{this.context.text}</div>;
        }
      }
      Component.contextTypes = {
        text: PropTypes.string.isRequired,
      };
      class ContextProvider extends React.Component {
        getChildContext() {
          return {
            text: 'hello, world',
          };
        }
        render() {
          return this.props.children;
        }
      }
      ContextProvider.childContextTypes = {
        text: PropTypes.string,
      };
      var markup = ReactDOMServer.renderToString(
        <ContextProvider><Component /></ContextProvider>,
      );
      expect(markup).toContain('hello, world');
    });
    it('renders components with different batching strategies', () => {
      class StaticComponent extends React.Component {