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**
constructor(options) {
    super(options);
    this.buffer = '';
  }
  _write(chunk, encoding, cb) {
    this.buffer += chunk;
    cb();
  }
}
async function renderIntoStream(reactElement, errorCount = 0) {
  return await expectErrors(
    () =>
      new Promise(resolve => {
        let writable = new DrainWritable();
        ReactDOMNodeStream.renderToStream(reactElement).pipe(writable);
        writable.on('finish', () => resolve(writable.buffer));
      }),
    errorCount,
  );
}
async function streamRender(reactElement, errorCount = 0) {