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**
render() {
        var inner = (
          <Wrapper object={innerObj} ref={c => (this.innerRef = c)} />
        );
        var outer = (
          <Wrapper object={outerObj} ref={c => (this.outerRef = c)}>
            {inner}
          </Wrapper>
        );
        return outer;
      }
      componentDidMount() {
        expect(this.innerRef.getObject()).toEqual(innerObj);
        expect(this.outerRef.getObject()).toEqual(outerObj);
        mounted = true;
      }
    }
    ReactTestUtils.renderIntoDocument(<Component />);
    expect(mounted).toBe(true);
  });
  it('should support new-style refs with mixed-up owners', () => {
    class Wrapper extends React.Component {