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**
resetCurrentDebugStack();
      }
    }
    return out;
  }
  render(child, context) {
    if (typeof child === 'string' || typeof child === 'number') {
      var text = '' + child;
      if (text === '') {
        return '';
      }
      if (this.makeStaticMarkup) {
        return escapeTextContentForBrowser(text);
      }
      if (this.previousWasTextNode) {
        return '<!-- -->' + escapeTextContentForBrowser(text);
      }
      this.previousWasTextNode = true;
      return escapeTextContentForBrowser(text);
    } else {
      ({child, context} = resolve(child, context));
      if (child === null || child === false) {
        return '';
      } else {
        return this.renderDOM(child, context);
      }
    }
  }
  renderDOM(element, context) {