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**
/**
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule ReactDOMStringRenderer
 */
'use strict';
var invariant = require('fbjs/lib/invariant');
var React = require('react');
var ReactPartialRenderer = require('ReactPartialRenderer');
/**
 * Render a ReactElement to its initial HTML. This should only be used on the
 * server.
 * See https://facebook.github.io/react/docs/react-dom-server.html#rendertostring
 */
function renderToString(element) {