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**
<div style={{color: undefined, width: '30px'}} />,
        );
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('30px');
      });
      itRenders('no null styles', async render => {
        const e = await render(<div style={{color: null, width: '30px'}} />);
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('30px');
      });
      itRenders('no empty styles', async render => {
        const e = await render(<div style={{color: null, width: null}} />);
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('');
        expect(e.hasAttribute('style')).toBe(false);
      });
    });
    describe('aria attributes', function() {
      itRenders('simple strings', async render => {
        const e = await render(<div aria-label="hello" />);
        expect(e.getAttribute('aria-label')).toBe('hello');
      });
      
      itRenders('aria string prop with false value', async render => {
        const e = await render(<div aria-label={false} />);
        expect(e.getAttribute('aria-label')).toBe('false');
      });
      itRenders('no aria prop with null value', async render => {
        const e = await render(<div aria-label={null} />);
        expect(e.hasAttribute('aria-label')).toBe(false);