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 contextTypes = type.contextTypes;
  if (!contextTypes) {
    return emptyObject;
  }
  var maskedContext = {};
  for (var contextName in contextTypes) {
    maskedContext[contextName] = context[contextName];
  }
  return maskedContext;
}
function checkContextTypes(typeSpecs, values, location: string) {