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>;
        }
      }
      const e = await render(
        <PurpleContext><ClassChildWithoutContext /></PurpleContext>,
      );
      expect(e.textContent).toBe('');
    });
    itRenders('stateless child without context', async render => {
      function StatelessChildWithoutContext(props, context) {
        
        return <div>{context.text}</div>;
      }
      const e = await render(
        <PurpleContext><StatelessChildWithoutContext /></PurpleContext>,
      );
      expect(e.textContent).toBe('');
    });
    itRenders('class child with wrong context', async render => {
      class ClassChildWithWrongContext extends React.Component {