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>{this.context.text}</div>;
        }
      }
      ClassChildWithContext.contextTypes = {text: PropTypes.string};
      const e = await render(
        <PurpleContext><ClassChildWithContext /></PurpleContext>,
      );
      expect(e.textContent).toBe('purple');
    });
    itRenders('stateless child with context', async render => {
      function StatelessChildWithContext(props, context) {
        return <div>{context.text}</div>;
      }
      StatelessChildWithContext.contextTypes = {text: PropTypes.string};
      const e = await render(
        <PurpleContext><StatelessChildWithContext /></PurpleContext>,
      );
      expect(e.textContent).toBe('purple');
    });
    itRenders('class child without context', async render => {
      class ClassChildWithoutContext extends React.Component {