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**
lifecycle.push('componentWillReceiveProps');
          }
          componentWillUnmount() {
            lifecycle.push('componentWillUnmount');
          }
        }
        var response = ReactDOMServer.renderToString(<TestComponent />);
        expect(response).toMatch(
          new RegExp(
            '<span ' +
              ROOT_ATTRIBUTE_NAME +
              '="" ' +
              ID_ATTRIBUTE_NAME +
              '="[^"]*"' +
              (ReactDOMFeatureFlags.useFiber
                ? ''
                : ' ' + ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+"') +
              '>' +
              (ReactDOMFeatureFlags.useFiber
                ? 'Component name: <!-- -->TestComponent'
                : '<!-- react-text: [0-9]+ -->Component name: <!-- /react-text -->' +
                    '<!-- react-text: [0-9]+ -->TestComponent<!-- /react-text -->') +
              '</span>',
          ),
        );
        expect(lifecycle).toEqual([
          'getInitialState',
          'componentWillMount',
          'render',
        ]);