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**
super();
            this.state = {value: 'Hello'};
          }
          handleChange(event) {
            if (this.props.onChange) {
              this.props.onChange(event);
            }
            this.setState({value: event.target.value});
          }
          render() {
            return (
              <textarea
                value={this.state.value}
                onChange={this.handleChange.bind(this)}
              />
            );
          }
        };
        ControlledCheckbox = class extends React.Component {
          constructor() {
            super();
            this.state = {value: true};
          }
          handleChange(event) {