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**
lifecycle.push('componentWillUnmount');
          }
        }
        var response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
        expect(response).toBe('<span>Component name: TestComponent</span>');
        expect(lifecycle).toEqual([
          'getInitialState',
          'componentWillMount',
          'render',
        ]);
      }
      runTest();
      
      ExecutionEnvironment.canUseDOM = true;
      runTest();
    });
    it('should throw with silly args', () => {
      expect(
        ReactDOMServer.renderToStaticMarkup.bind(
          ReactDOMServer,
          'not a component',
        ),
      ).toThrowError(
        'renderToStaticMarkup(): You must pass a valid ReactElement.',
      );
    });
    it('allows setState in componentWillMount without using DOM', () => {
      class Component extends React.Component {