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**
itRenders(
        'a newline-eating tag with content not starting with \\n',
        async render => {
          const e = await render(<pre>Hello</pre>);
          expect(e.textContent).toBe('Hello');
        },
      );
      itRenders(
        'a newline-eating tag with content starting with \\n',
        async render => {
          const e = await render(<pre>{'\nHello'}</pre>);
          expect(e.textContent).toBe('\nHello');
        },
      );
      itRenders('a normal tag with content starting with \\n', async render => {
        const e = await render(<div>{'\nHello'}</div>);
        expect(e.textContent).toBe('\nHello');
      });
    });
    describe('different component implementations', function() {
      function checkFooDiv(e) {
        expect(e.childNodes.length).toBe(1);
        expectNode(e.firstChild, TEXT_NODE_TYPE, 'foo');
      }
      itRenders('stateless components', async render => {
        const StatelessComponent = () => <div>foo</div>;
        checkFooDiv(await render(<StatelessComponent />));
      });
      itRenders('ES6 class components', async render => {
        class ClassComponent extends React.Component {