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**
it('should reconnect Pure Component to Pure Component', () =>
          expectMarkupMatch(
            <PureComponent id="foobarbaz" />,
            <PureComponent id="foobarbaz" />,
          ));
        it('should reconnect Bare Element to Pure Component', () =>
          expectMarkupMatch(<PureComponent id="foobarbaz" />, bareElement));
        it('should reconnect ES6 Class to Bare Element', () =>
          expectMarkupMatch(bareElement, <ES6ClassComponent id="foobarbaz" />));
        it('should reconnect Pure Component to Bare Element', () =>
          expectMarkupMatch(bareElement, <PureComponent id="foobarbaz" />));
        it('should reconnect Bare Element to Bare Element', () =>
          expectMarkupMatch(bareElement, bareElement));
      });
      it('should error reconnecting different element types', () =>
        expectMarkupMismatch(<div />, <span />));
      it('should error reconnecting missing attributes', () =>
        expectMarkupMismatch(<div id="foo" />, <div />));
      it('should error reconnecting added attributes', () =>
        expectMarkupMismatch(<div />, <div id="foo" />));
      it('should error reconnecting different attribute values', () =>
        expectMarkupMismatch(<div id="foo" />, <div id="bar" />));
    });
    describe('inline styles', function() {
      it('should error reconnecting missing style attribute', () =>
        expectMarkupMismatch(<div style={{width: '1px'}} />, <div />));
      it('should error reconnecting added style attribute', () =>
        expectMarkupMismatch(<div />, <div style={{width: '1px'}} />));
      it('should error reconnecting empty style attribute', () =>
        expectMarkupMismatch(