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(node).not.toBe(null);
      expect(node.nodeType).toBe(type);
      expect(node.nodeValue).toMatch(value);
    }
    function expectTextNode(node, text) {
      if (ReactDOMFeatureFlags.useFiber) {
        
        expectNode(node, TEXT_NODE_TYPE, text);
      } else {
        
        
        expectNode(node, COMMENT_NODE_TYPE, / react-text: [0-9]+ /);
        if (text.length > 0) {
          node = node.nextSibling;
          expectNode(node, TEXT_NODE_TYPE, text);
        }
        expectNode(node.nextSibling, COMMENT_NODE_TYPE, / \/react-text /);
      }
    }
    function expectEmptyNode(node) {