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**
return <div>{context.text}</div>;
      }
      Grandchild.contextTypes = {text: PropTypes.string};
      const Child = props => <Grandchild />;
      const e = await render(<PurpleContext><Child /></PurpleContext>);
      expect(e.textContent).toBe('purple');
    });
    itRenders('a child context overriding a parent context', async render => {
      const Grandchild = (props, context) => {
        return <div>{context.text}</div>;
      };
      Grandchild.contextTypes = {text: PropTypes.string};
      const e = await render(
        <PurpleContext><RedContext><Grandchild /></RedContext></PurpleContext>,
      );
      expect(e.textContent).toBe('red');
    });
    itRenders('a child context merged with a parent context', async render => {
      class Parent extends React.Component {
        getChildContext() {
          return {text1: 'purple'};
        }
        render() {
          return <Child />;
        }
      }
      Parent.childContextTypes = {text1: PropTypes.string};
      class Child extends React.Component {