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**
expect(e.hasAttribute('foo')).toBe(false);
        },
      );
      itRenders(
        'unknown attributes for custom elements using is',
        async render => {
          const e = await render(<div is="custom-element" foo="bar" />);
          expect(e.getAttribute('foo')).toBe('bar');
        },
      );
      itRenders(
        'no unknown attributes for custom elements using is with null value',
        async render => {
          const e = await render(<div is="custom-element" foo={null} />);
          expect(e.hasAttribute('foo')).toBe(false);
        },
      );
    });
    itRenders('no HTML events', async render => {
      const e = await render(<div onClick={() => {}} />);
      expect(e.getAttribute('onClick')).toBe(null);
      expect(e.getAttribute('onClick')).toBe(null);
      expect(e.getAttribute('click')).toBe(null);
    });
  });
  describe('elements and children', function() {
    
    const TEXT_NODE_TYPE = 3;
    const COMMENT_NODE_TYPE = 8;
    function expectNode(node, type, value) {