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**
expect(e.childNodes.length).toBe(3);
            child1 = e.childNodes[0];
            textNode = e.childNodes[1];
            child2 = e.childNodes[2];
          } else {
            
            
            expect(e.childNodes.length).toBe(5);
            child1 = e.childNodes[0];
            textNode = e.childNodes[1];
            child2 = e.childNodes[4];
          }
          expect(child1.id).toBe('child1');
          expect(child1.childNodes.length).toBe(0);
          expectTextNode(textNode, ' ');
          expect(child2.id).toBe('child2');
          expect(child2.childNodes.length).toBe(0);
        },
      );
      itRenders(
        'a div with a single child surrounded by whitespace',
        async render => {
          
          const e = await render(<div id="parent">  <div id="child" />   </div>); 
          let textNode1, child, textNode2;
          if (ReactDOMFeatureFlags.useFiber) {
            
            
            expect(e.childNodes.length).toBe(3);