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**
expectNode(node, COMMENT_NODE_TYPE, / react-empty: [0-9]+ /);
    }
    describe('text children', function() {
      itRenders('a div with text', async render => {
        const e = await render(<div>Text</div>);
        expect(e.tagName).toBe('DIV');
        expect(e.childNodes.length).toBe(1);
        expectNode(e.firstChild, TEXT_NODE_TYPE, 'Text');
      });
      itRenders('a div with text with flanking whitespace', async render => {
        
        const e = await render(<div>  Text </div>);
        expect(e.childNodes.length).toBe(1);
        expectNode(e.childNodes[0], TEXT_NODE_TYPE, '  Text ');
      });
      itRenders('a div with an empty text child', async render => {
        const e = await render(<div>{''}</div>);
        expect(e.childNodes.length).toBe(0);
      });
      itRenders('a div with multiple empty text children', async render => {
        const e = await render(<div>{''}{''}{''}</div>);
        if (ReactDOMFeatureFlags.useFiber) {
          
          
          if (render === serverRender || render === streamRender) {