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**
var serialized = '';
  var delimiter = '';
  for (var styleName in styles) {
    if (!styles.hasOwnProperty(styleName)) {
      continue;
    }
    var isCustomProperty = styleName.indexOf('--') === 0;
    var styleValue = styles[styleName];
    if (__DEV__) {
      if (!isCustomProperty) {
        warnValidStyle(styleName, styleValue, component);
      }
    }
    if (styleValue != null) {
      serialized += delimiter + processStyleName(styleName) + ':';
      serialized += dangerousStyleValue(
        styleName,
        styleValue,
        isCustomProperty,
      );
      delimiter = ';';
    }
  }
  return serialized || null;
}
function warnNoop(