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__) {
    checkPropTypes(typeSpecs, values, location, 'Component', getStackAddendum);
  }
}
function processContext(type, context) {
  var maskedContext = maskContext(type, context);
  if (__DEV__) {
    if (type.contextTypes) {
      checkContextTypes(type.contextTypes, maskedContext, 'context');
    }
  }
  return maskedContext;
}
var STYLE = 'style';
var RESERVED_PROPS = {
  children: null,
  dangerouslySetInnerHTML: null,
  suppressContentEditableWarning: null,
};
function isCustomComponent(tagName, props) {