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**
return tagName.indexOf('-') >= 0 || props.is != null;
}
function createOpenTagMarkup(
  tagVerbatim,
  tagLowercase,
  props,
  makeStaticMarkup,
  isRootElement,
  instForDebug,
) {
  var ret = '<' + tagVerbatim;
  for (var propKey in props) {
    if (!props.hasOwnProperty(propKey)) {
      continue;
    }
    var propValue = props[propKey];
    if (propValue == null) {
      continue;
    }
    if (!registrationNameModules.hasOwnProperty(propKey)) {
      if (propKey === STYLE) {
        propValue = createMarkupForStyles(propValue, instForDebug);
      }
      var markup = null;