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**
ReactReconcileTransaction = require('ReactReconcileTransaction');
    PropTypes = require('prop-types');
    ExecutionEnvironment = require('fbjs/lib/ExecutionEnvironment');
    ExecutionEnvironment.canUseDOM = false;
    ReactDOMServer = require('react-dom/server');
    var DOMProperty = require('DOMProperty');
    ID_ATTRIBUTE_NAME = DOMProperty.ID_ATTRIBUTE_NAME;
    ROOT_ATTRIBUTE_NAME = DOMProperty.ROOT_ATTRIBUTE_NAME;
  });
  describe('renderToString', () => {
    it('should generate simple markup', () => {
      var response = ReactDOMServer.renderToString(<span>hello world</span>);
      expect(response).toMatch(
        new RegExp(
          '<span ' +
            ROOT_ATTRIBUTE_NAME +
            '="" ' +
            ID_ATTRIBUTE_NAME +
            '="[^"]*"' +
            (ReactDOMFeatureFlags.useFiber
              ? ''
              : ' ' + ReactMarkupChecksum.CHECKSUM_ATTR_NAME + '="[^"]+"') +
            '>hello world</span>',
        ),
      );
    });
    it('should generate simple markup for self-closing tags', () => {
      var response = ReactDOMServer.renderToString(<img />);
      expect(response).toMatch(
        new RegExp(