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**
child = null;
      }
    }
    var childContext = inst.getChildContext && inst.getChildContext();
    if (childContext) {
      context = Object.assign({}, context, childContext);
    }
  }
  return {child, context};
}
class ReactDOMServerRenderer {
  constructor(element, makeStaticMarkup) {
    var topFrame = {
      children: [element],
      childIndex: 0,
      context: emptyObject,
      footer: '',
    };
    if (__DEV__) {
      topFrame.debugElementStack = [];
    }
    this.stack = [topFrame];
    this.exhausted = false;
    this.currentSelectValue = null;
    this.previousWasTextNode = false;
    this.makeStaticMarkup = makeStaticMarkup;
  }
  read(bytes) {