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 invariant = require('fbjs/lib/invariant');
var memoizeStringOnly = require('fbjs/lib/memoizeStringOnly');
var omittedCloseTags = require('omittedCloseTags');
var toArray = React.Children.toArray;
if (__DEV__) {
  var warning = require('fbjs/lib/warning');
  var checkPropTypes = require('prop-types/checkPropTypes');
  var warnValidStyle = require('warnValidStyle');
  var {
    validateProperties: validateARIAProperties,
  } = require('ReactDOMInvalidARIAHook');
  var {
    validateProperties: validateInputPropertes,
  } = require('ReactDOMNullInputValuePropHook');
  var {
    validateProperties: validateUnknownPropertes,
  } = require('ReactDOMUnknownPropertyHook');
  var validatePropertiesInDevelopment = function(type, props) {
    validateARIAProperties(type, props);
    validateInputPropertes(type, props);
    validateUnknownPropertes(type, props);
  };
  var describeComponentFrame = require('describeComponentFrame');
  var describeStackFrame = function(element): string {
    var source = element._source;
    var type = element.type;
    var name = typeof type === 'string'
      ? type
      : typeof type === 'function' ? type.displayName || type.name : null;
    var ownerName = null;