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**
itRenders('boolean prop with false value', async render => {
        const e = await render(<div hidden={false} />);
        expect(e.getAttribute('hidden')).toBe(null);
      });
      itRenders('boolean prop with self value', async render => {
        const e = await render(<div hidden="hidden" />);
        expect(e.getAttribute('hidden')).toBe('');
      });
      
      
      
      itRenders('boolean prop with "" value', async render => {
        const e = await render(<div hidden="" />);
        expect(e.getAttribute('hidden')).toBe(null);
      });
      
      itRenders('boolean prop with string value', async render => {
        const e = await render(<div hidden="foo" />);
        expect(e.getAttribute('hidden')).toBe('');
      });
      
      itRenders('boolean prop with array value', async render => {
        const e = await render(<div hidden={['foo', 'bar']} />);
        expect(e.getAttribute('hidden')).toBe('');
      });
      
      itRenders('boolean prop with object value', async render => {
        const e = await render(<div hidden={{foo: 'bar'}} />);
        expect(e.getAttribute('hidden')).toBe('');
      });