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 null;
        }
      };
    });
    describe('elements', function() {
      describe('reconnecting different component implementations', function() {
        let ES6ClassComponent, PureComponent, bareElement;
        beforeEach(() => {
          
          ES6ClassComponent = class extends React.Component {
            render() {