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**
expect(e.tagName).toBe('DIV');
          expect(e.childNodes.length).toBe(1);
          e = e.firstChild;
        }
        expect(e.tagName).toBe('DIV');
        expect(e.childNodes.length).toBe(0);
      });
      itRenders('multi-child hierarchies of components', async render => {
        const Component = props => <div>{props.children}</div>;
        const e = await render(
          <Component>
            <Component>
              <Component /><Component />
            </Component>
            <Component>
              <Component /><Component />
            </Component>
          </Component>,
        );
        expect(e.tagName).toBe('DIV');
        expect(e.childNodes.length).toBe(2);
        for (var i = 0; i < 2; i++) {
          var child = e.childNodes[i];
          expect(child.tagName).toBe('DIV');
          expect(child.childNodes.length).toBe(2);
          for (var j = 0; j < 2; j++) {
            var grandchild = child.childNodes[j];
            expect(grandchild.tagName).toBe('DIV');
            expect(grandchild.childNodes.length).toBe(0);
          }