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.getAttribute('for')).toBe('false');
      });
      itRenders('no htmlFor prop with null value', async render => {
        const e = await render(<div htmlFor={null} />);
        expect(e.hasAttribute('htmlFor')).toBe(false);
      });
    });
    describe('numeric properties', function() {
      itRenders(
        'positive numeric property with positive value',
        async render => {
          const e = await render(<input size={2} />);
          expect(e.getAttribute('size')).toBe('2');
        },
      );
      itRenders(
        'no positive numeric property with zero value',
        async render => {
          const e = await render(<input size={0} />);
          expect(e.hasAttribute('size')).toBe(false);
        },
      );
      itRenders('numeric property with zero value', async render => {
        const e = await render(<ol start={0} />);
        expect(e.getAttribute('start')).toBe('0');
      });
    });
    describe('props with special meaning in React', function() {
      itRenders('no ref attribute', async render => {
        class RefComponent extends React.Component {