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 tag = element.type.toLowerCase();
    if (__DEV__) {
      warning(
        tag === element.type,
        '<%s /> is using uppercase HTML. Always use lowercase HTML tags ' +
          'in React.',
        element.type,
      );
    }
    validateDangerousTag(tag);
    var props = element.props;
    if (tag === 'input') {
      if (__DEV__) {
        ReactControlledValuePropTypes.checkPropTypes(
          'input',
          props,
          () => '', 
        );
        if (
          props.checked !== undefined &&
          props.defaultChecked !== undefined &&
          !didWarnDefaultChecked
        ) {
          warning(
            false,
            '%s contains an input of type %s with both checked and defaultChecked props. ' +
              'Input elements must be either controlled or uncontrolled ' +
              '(specify either the checked prop, or the defaultChecked prop, but not ' +
              'both). Decide between using a controlled or uncontrolled input ' +
              'element and remove one of these props. More info: ' +