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**
'<img ' +
            ROOT_ATTRIBUTE_NAME +
            '="" ' +
            ID_ATTRIBUTE_NAME +
            '="[^"]*"' +
            (ReactDOMFeatureFlags.useFiber
              ? ''
              : ' ' + ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+"') +
            '/>',
        ),
      );
    });
    it('should generate simple markup for attribute with `>` symbol', () => {
      var response = ReactDOMServer.renderToString(<img data-attr=">" />);
      expect(response).toMatch(
        new RegExp(
          '<img data-attr="&gt;" ' +
            ROOT_ATTRIBUTE_NAME +
            '="" ' +
            ID_ATTRIBUTE_NAME +
            '="[^"]*"' +
            (ReactDOMFeatureFlags.useFiber
              ? ''
              : ' ' + ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+"') +
            '/>',
        ),
      );
    });
    it('should generate comment markup for component returns null', () => {
      class NullComponent extends React.Component {