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 <span>My name is {this.props.name}</span>;
        }
      }
      var response = ReactDOMServer.renderToString(<Parent />);
      expect(response).toMatch(
        new RegExp(
          '<div ' +
            ROOT_ATTRIBUTE_NAME +
            '="" ' +
            ID_ATTRIBUTE_NAME +
            '="[^"]*"' +
            (ReactDOMFeatureFlags.useFiber
              ? ''
              : ' ' + ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+"') +
            '>' +
            '<span ' +
            ID_ATTRIBUTE_NAME +
            '="[^"]*">' +
            (ReactDOMFeatureFlags.useFiber
              ? 'My name is <!-- -->child'
              : '<!-- react-text: [0-9]+ -->My name is <!-- /react-text -->' +
                  '<!-- react-text: [0-9]+ -->child<!-- /react-text -->') +
            '</span>' +
            '</div>',
        ),
      );
    });
    it('should only execute certain lifecycle methods', () => {
      function runTest() {