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(<textarea defaultValue="foo" />);
        expect(e.getAttribute('value')).toBe(null);
        expect(e.getAttribute('defaultValue')).toBe(null);
        expect(e.value).toBe('foo');
      });
      itRenders('a textarea value overriding defaultValue', async render => {
        const e = await render(
          <textarea value="foo" defaultValue="bar" readOnly={true} />,
          1,
        );
        expect(e.getAttribute('value')).toBe(null);
        expect(e.getAttribute('defaultValue')).toBe(null);
        expect(e.value).toBe('foo');
      });
      itRenders(
        'a textarea value overriding defaultValue no matter the prop order',
        async render => {
          const e = await render(
            <textarea defaultValue="bar" value="foo" readOnly={true} />,
            1,
          );
          expect(e.getAttribute('value')).toBe(null);
          expect(e.getAttribute('defaultValue')).toBe(null);
          expect(e.value).toBe('foo');
        },
      );
    });
    describe('selects', function() {
      var options;
      beforeEach(function() {