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**
<div style={{width: '1px'}} />,
          <div style={{}} />,
        ));
      it('should error reconnecting added style values', () =>
        expectMarkupMismatch(
          <div style={{}} />,
          <div style={{width: '1px'}} />,
        ));
      it('should error reconnecting different style values', () =>
        expectMarkupMismatch(
          <div style={{width: '1px'}} />,
          <div style={{width: '2px'}} />,
        ));
      it('should reconnect number and string versions of a number', () =>
        expectMarkupMatch(
          <div style={{width: '1px', height: 2}} />,
          <div style={{width: 1, height: '2px'}} />,
        ));
      it('should error reconnecting reordered style values', () =>
        expectMarkupMismatch(
          <div style={{width: '1px', fontSize: '2px'}} />,
          <div style={{fontSize: '2px', width: '1px'}} />,
        ));
    });
    describe('text nodes', function() {
      it('should error reconnecting different text', () =>
        expectMarkupMismatch(<div>Text</div>, <div>Other Text</div>));
      it('should reconnect a div with a number and string version of number', () =>
        expectMarkupMatch(<div>{2}</div>, <div>2</div>));
      it('should error reconnecting different numbers', () =>