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**
props.children[1] = <p key={1} />; 
      return <div>{props.children}</div>;
    }
    expect(() => {
      ReactDOMServer.renderToStaticMarkup(
        <Wrapper>
          <span key={0} />
          <span key={1} />
          <span key={2} />
        </Wrapper>,
      );
    }).toThrowError(/Cannot assign to read only property.*/);
  });
});