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 />;
          }
          getChildContext() {
            return {foo: 'bar'};
          }
        }
        return render(<Component />);
      },
    );
    itThrowsWhenRendering(
      'if getChildContext returns a value not in childContextTypes',
      render => {
        class Component extends React.Component {
          render() {
            return <div />;
          }
          getChildContext() {
            return {value1: 'foo', value2: 'bar'};
          }
        }
        Component.childContextTypes = {value1: PropTypes.string};
        return render(<Component />);
      },
    );
  });
  describe('refs', function() {
    it('should not run ref code on server', async () => {
      let refCount = 0;
      class RefsComponent extends React.Component {