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**
render() {
          return <span><NestedComponent /></span>;
        }
      }
      var response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
      expect(response).toBe('<span><div>inner text</div></span>');
    });
    it('should not put checksum and React ID on text components', () => {
      class TestComponent extends React.Component {
        render() {
          return <span>{'hello'} {'world'}</span>;
        }
      }
      var response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
      expect(response).toBe('<span>hello world</span>');
    });
    it('should not use comments for empty nodes', () => {
      class TestComponent extends React.Component {
        render() {
          return null;
        }
      }
      var response = ReactDOMServer.renderToStaticMarkup(<TestComponent />);
      expect(response).toBe('');
    });
    it('should only execute certain lifecycle methods', () => {
      function runTest() {
        var lifecycle = [];
        class TestComponent extends React.Component {