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**
out += '\n';
      }
      out += innerMarkup;
    } else {
      children = toArray(props.children);
    }
    var frame = {
      tag,
      children,
      childIndex: 0,
      context: context,
      footer: footer,
    };
    if (__DEV__) {
      frame.debugElementStack = [];
    }
    this.stack.push(frame);
    return out;
  }
}
module.exports = ReactDOMServerRenderer;