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**
getObject = () => {
        return this.props.object;
      };
      render() {
        return <div>{this.props.children}</div>;
      }
    }
    class Component extends React.Component {
      render() {
        var inner = <Wrapper object={innerObj} ref="inner" />;
        var outer = <Wrapper object={outerObj} ref="outer">{inner}</Wrapper>;
        return outer;
      }
      componentDidMount() {
        expect(this.refs.inner.getObject()).toEqual(innerObj);
        expect(this.refs.outer.getObject()).toEqual(outerObj);
      }
    }
    ReactTestUtils.renderIntoDocument(<Component />);
  });
  it('should not have refs on unmounted components', () => {
    class Parent extends React.Component {