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**
invariant(
    React.isValidElement(element),
    'renderToStream(): You must pass a valid ReactElement.',
  );
  return new ReactMarkupReadableStream(element, false);
}
/**
 * Similar to renderToStream, except this doesn't create extra DOM attributes
 * such as data-react-id that React uses internally.
 * See https://facebook.github.io/react/docs/react-dom-stream.html#rendertostaticstream
 */
function renderToStaticStream(element) {
  invariant(
    React.isValidElement(element),
    'renderToStaticStream(): You must pass a valid ReactElement.',
  );
  return new ReactMarkupReadableStream(element, true);
}
module.exports = {
  renderToStream: renderToStream,
  renderToStaticStream: renderToStaticStream,
};