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 non-zero number value', async render => {
        const e = await render(<div hidden={10} />);
        expect(e.getAttribute('hidden')).toBe('');
      });
      
      itRenders('boolean prop with zero value', async render => {
        const e = await render(<div hidden={0} />);
        expect(e.getAttribute('hidden')).toBe(null);
      });
      itRenders('no boolean prop with null value', async render => {
        const e = await render(<div hidden={null} />);
        expect(e.hasAttribute('hidden')).toBe(false);
      });
    });
    describe('download property (combined boolean/string attribute)', function() {
      itRenders('download prop with true value', async render => {
        const e = await render(<a download={true} />);
        expect(e.getAttribute('download')).toBe('');
      });
      itRenders('download prop with false value', async render => {
        const e = await render(<a download={false} />);
        expect(e.getAttribute('download')).toBe(null);
      });
      itRenders('download prop with string value', async render => {
        const e = await render(<a download="myfile" />);
        expect(e.getAttribute('download')).toBe('myfile');
      });
      itRenders('download prop with string "false" value', async render => {
        const e = await render(<a download="false" />);