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**
onAfterResetModules();
  }
  ReactDOMServer = require('react-dom/server');
  
  jest.resetModuleRegistry();
  if (typeof onAfterResetModules === 'function') {
    onAfterResetModules();
  }
  ReactDOMNodeStream = require('react-dom/node-stream');
}
describe('ReactDOMServerIntegration', () => {
  beforeEach(() => {
    resetModules();
    ExecutionEnvironment.canUseDOM = false;
  });
  describe('basic rendering', function() {
    itRenders('a blank div', async render => {
      const e = await render(<div />);
      expect(e.tagName).toBe('DIV');
    });
    itRenders('a self-closing tag', async render => {
      const e = await render(<br />);
      expect(e.tagName).toBe('BR');
    });
    itRenders('a self-closing tag as a child', async render => {
      const e = await render(<div><br /></div>);
      expect(e.childNodes.length).toBe(1);
      expect(e.firstChild.tagName).toBe('BR');
    });
  });