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**
});
    });
    describe('unknown attributes', function() {
      itRenders('no unknown attributes', async render => {
        const e = await render(<div foo="bar" />, 1);
        expect(e.getAttribute('foo')).toBe(null);
      });
      itRenders('unknown data- attributes', async render => {
        const e = await render(<div data-foo="bar" />);
        expect(e.getAttribute('data-foo')).toBe('bar');
      });
      itRenders('no unknown data- attributes with null value', async render => {
        const e = await render(<div data-foo={null} />);
        expect(e.hasAttribute('data-foo')).toBe(false);
      });
      itRenders(
        'no unknown attributes for non-standard elements',
        async render => {
          const e = await render(<nonstandard foo="bar" />, 1);
          expect(e.getAttribute('foo')).toBe(null);
        },
      );
      itRenders('unknown attributes for custom elements', async render => {
        const e = await render(<custom-element foo="bar" />);
        expect(e.getAttribute('foo')).toBe('bar');
      });
      itRenders(
        'no unknown attributes for custom elements with null value',
        async render => {
          const e = await render(<custom-element foo={null} />);