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**
for (let ii = debugElementStack.length - 1; ii >= 0; ii--) {
        stack += describeStackFrame(debugElementStack[ii]);
      }
    }
    return stack;
  };
}
var didWarnDefaultInputValue = false;
var didWarnDefaultChecked = false;
var didWarnDefaultSelectValue = false;
var didWarnDefaultTextareaValue = false;
var didWarnInvalidOptionChildren = false;
var valuePropNames = ['value', 'defaultValue'];
var newlineEatingTags = {
  listing: true,
  pre: true,
  textarea: true,
};
var VALID_TAG_REGEX = /^[a-zA-Z][a-zA-Z:_\.\-\d]*$/; 
var validatedTagCache = {};
function validateDangerousTag(tag) {
  if (!validatedTagCache.hasOwnProperty(tag)) {
    invariant(VALID_TAG_REGEX.test(tag), 'Invalid tag: %s', tag);
    validatedTagCache[tag] = true;
  }
}
var processStyleName = memoizeStringOnly(function(styleName) {
  return hyphenateStyleName(styleName);
});
function createMarkupForStyles(styles, component) {