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 id="classWrongChild">{this.context.text}</div>;
        }
      }
      ClassChildWithWrongContext.contextTypes = {foo: PropTypes.string};
      const e = await render(
        <PurpleContext><ClassChildWithWrongContext /></PurpleContext>,
      );
      expect(e.textContent).toBe('');
    });
    itRenders('stateless child with wrong context', async render => {
      function StatelessChildWithWrongContext(props, context) {
        
        return <div id="statelessWrongChild">{context.text}</div>;
      }
      StatelessChildWithWrongContext.contextTypes = {
        foo: PropTypes.string,
      };
      const e = await render(
        <PurpleContext><StatelessChildWithWrongContext /></PurpleContext>,
      );
      expect(e.textContent).toBe('');
    });
    itRenders('with context passed through to a grandchild', async render => {
      function Grandchild(props, context) {