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**
if (this.exhausted) {
      return null;
    }
    var out = '';
    while (out.length < bytes) {
      if (this.stack.length === 0) {
        this.exhausted = true;
        break;
      }
      var frame = this.stack[this.stack.length - 1];
      if (frame.childIndex >= frame.children.length) {
        out += frame.footer;
        this.previousWasTextNode = false;
        this.stack.pop();
        if (frame.tag === 'select') {
          this.currentSelectValue = null;
        }
        continue;
      }
      var child = frame.children[frame.childIndex++];
      if (__DEV__) {
        setCurrentDebugStack(this.stack);
      }
      out += this.render(child, frame.context);
      if (__DEV__) {