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 <div ref={e => refCount++} />;
        }
      }
      await expectMarkupMatch(<RefsComponent />, <div />);
      expect(refCount).toBe(0);
    });
    it('should run ref code on client', async () => {
      let refCount = 0;
      class RefsComponent extends React.Component {
        render() {
          return <div ref={e => refCount++} />;
        }
      }
      await expectMarkupMatch(<div />, <RefsComponent />);
      expect(refCount).toBe(1);
    });
    it('should send the correct element to ref functions on client', async () => {
      let refElement = null;
      class RefsComponent extends React.Component {
        render() {
          return <div ref={e => (refElement = e)} />;
        }
      }
      const e = await clientRenderOnServerString(<RefsComponent />);
      expect(refElement).not.toBe(null);
      expect(refElement).toBe(e);
    });
    it('should have string refs on client when rendered over server markup', async () => {
      class RefsComponent extends React.Component {