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('download')).toBe('false');
      });
      itRenders('download prop with string "true" value', async render => {
        const e = await render(<a download={'true'} />);
        expect(e.getAttribute('download')).toBe('true');
      });
      itRenders('download prop with number 0 value', async render => {
        const e = await render(<a download={0} />);
        expect(e.getAttribute('download')).toBe('0');
      });
      itRenders('no download prop with null value', async render => {
        const e = await render(<div download={null} />);
        expect(e.hasAttribute('download')).toBe(false);
      });
      itRenders('no download prop with undefined value', async render => {
        const e = await render(<div download={undefined} />);
        expect(e.hasAttribute('download')).toBe(false);
      });
    });
    describe('className property', function() {
      itRenders('className prop with string value', async render => {
        const e = await render(<div className="myClassName" />);
        expect(e.getAttribute('class')).toBe('myClassName');
      });
      itRenders('className prop with empty string value', async render => {
        const e = await render(<div className="" />);
        expect(e.getAttribute('class')).toBe('');
      });
      
      itRenders('className prop with true value', async render => {