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**
describe('property to attribute mapping', function() {
    describe('string properties', function() {
      itRenders('simple numbers', async render => {
        const e = await render(<div width={30} />);
        expect(e.getAttribute('width')).toBe('30');
      });
      itRenders('simple strings', async render => {
        const e = await render(<div width={'30'} />);
        expect(e.getAttribute('width')).toBe('30');
      });
      
      itRenders('string prop with true value', async render => {
        const e = await render(<a href={true} />);
        expect(e.getAttribute('href')).toBe('true');
      });
      
      itRenders('string prop with false value', async render => {
        const e = await render(<a href={false} />);
        expect(e.getAttribute('href')).toBe('false');
      });
      itRenders('no string prop with null value', async render => {
        const e = await render(<div width={null} />);
        expect(e.hasAttribute('width')).toBe(false);
      });
    });
    describe('boolean properties', function() {
      itRenders('boolean prop with true value', async render => {
        const e = await render(<div hidden={true} />);
        expect(e.getAttribute('hidden')).toBe('');
      });