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**
render() {
        return (
          <div>
            <Inner
              id={1}
              ref={c => {
                log.push(`ref 1 got ${c ? `instance ${c.props.id}` : 'null'}`);
              }}
            />
            <Inner
              id={2}
              ref={c => {
                log.push(`ref 2 got ${c ? `instance ${c.props.id}` : 'null'}`);
              }}
            />
          </div>
        );
      }
      componentDidMount() {