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() {
        return <Child><div ref="test" /></Child>;
      }
      componentDidMount() {
        expect(this.refs && this.refs.test).toEqual(undefined);
      }
    }
    class Child extends React.Component {
      render() {
        return <div />;
      }
    }
    ReactTestUtils.renderIntoDocument(<Parent child={<span />} />);
  });
  it('should support new-style refs', () => {
    var innerObj = {};
    var outerObj = {};
    class Wrapper extends React.Component {
      getObject = () => {
        return this.props.object;
      };
      render() {
        return <div>{this.props.children}</div>;
      }
    }
    var mounted = false;
    class Component extends React.Component {