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**
var content = '';
  
  
  React.Children.forEach(children, function(child) {
    if (child == null) {
      return;
    }
    if (typeof child === 'string' || typeof child === 'number') {
      content += child;
    } else {
      if (__DEV__) {
        if (!didWarnInvalidOptionChildren) {
          didWarnInvalidOptionChildren = true;
          warning(
            false,
            'Only strings and numbers are supported as <option> children.',
          );
        }
      }
    }
  });
  return content;
}
function maskContext(type, context) {