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**
const UndefinedComponent = () => undefined;
        await render(<UndefinedComponent />, 1);
      });
      itThrowsWhenRendering('a number component', async render => {
        const NumberComponent = () => 54;
        await render(<NumberComponent />, 1);
      });
      itThrowsWhenRendering('null', render => render(null));
      itThrowsWhenRendering('false', render => render(false));
      itThrowsWhenRendering('undefined', render => render(undefined));
      itThrowsWhenRendering('number', render => render(30));
      itThrowsWhenRendering('string', render => render('foo'));
    });
  });
  describe('form controls', function() {
    describe('inputs', function() {
      itRenders('an input with a value and an onChange', async render => {
        const e = await render(<input value="foo" onChange={() => {}} />);
        expect(e.value).toBe('foo');
      });
      itRenders('an input with a value and readOnly', async render => {
        const e = await render(<input value="foo" readOnly={true} />);
        expect(e.value).toBe('foo');
      });
      itRenders(
        'an input with a value and no onChange/readOnly',
        async render => {
          
          
          const e = await render(<input value="foo" />, 1);