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 (__DEV__) {
      validatePropertiesInDevelopment(tag, props);
    }
    assertValidProps(tag, props);
    var out = createOpenTagMarkup(
      element.type,
      tag,
      props,
      this.makeStaticMarkup,
      this.stack.length === 1,
      null,
    );
    var footer = '';
    if (omittedCloseTags.hasOwnProperty(tag)) {
      out += '/>';
    } else {
      out += '>';
      footer = '</' + element.type + '>';
    }
    var children;
    var innerMarkup = getNonChildrenInnerMarkup(props);
    if (innerMarkup != null) {
      children = [];
      if (newlineEatingTags[tag] && innerMarkup.charAt(0) === '\n') {