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**
if (isCustomComponent(tagLowercase, props)) {
        if (!RESERVED_PROPS.hasOwnProperty(propKey)) {
          markup = DOMMarkupOperations.createMarkupForCustomAttribute(
            propKey,
            propValue,
          );
        }
      } else {
        markup = DOMMarkupOperations.createMarkupForProperty(
          propKey,
          propValue,
        );
      }
      if (markup) {
        ret += ' ' + markup;
      }
    }
  }
  
  
  if (makeStaticMarkup) {
    return ret;
  }
  if (isRootElement) {
    ret += ' ' + DOMMarkupOperations.createMarkupForRoot();
  }
  ret += ' ' + DOMMarkupOperations.createMarkupForID('');
  return ret;
}
function resolve(child, context) {