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**
const e = await render(<div className={true} />);
        expect(e.getAttribute('class')).toBe('true');
      });
      
      itRenders('className prop with false value', async render => {
        const e = await render(<div className={false} />);
        expect(e.getAttribute('class')).toBe('false');
      });
      itRenders('no className prop with null value', async render => {
        const e = await render(<div className={null} />);
        expect(e.hasAttribute('className')).toBe(false);
      });
    });
    describe('htmlFor property', function() {
      itRenders('htmlFor with string value', async render => {
        const e = await render(<div htmlFor="myFor" />);
        expect(e.getAttribute('for')).toBe('myFor');
      });
      itRenders('htmlFor with an empty string', async render => {
        const e = await render(<div htmlFor="" />);
        expect(e.getAttribute('for')).toBe('');
      });
      
      itRenders('htmlFor prop with true value', async render => {
        const e = await render(<div htmlFor={true} />);
        expect(e.getAttribute('for')).toBe('true');
      });
      
      itRenders('htmlFor prop with false value', async render => {
        const e = await render(<div htmlFor={false} />);