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**
publicInstance: ReactComponent<any, any, any>,
  callerName: string,
) {
  if (__DEV__) {
    var constructor = publicInstance.constructor;
    warning(
      false,
      '%s(...): Can only update a mounting component. ' +
        'This usually means you called %s() outside componentWillMount() on the server. ' +
        'This is a no-op.\n\nPlease check the code for the %s component.',
      callerName,
      callerName,
      (constructor && (constructor.displayName || constructor.name)) ||
        'ReactClass',
    );
  }
}
function shouldConstruct(Component) {