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() {
        var children = {
          a: <span />,
          b: <span />,
          c: <span />,
        };
        return <div>{[children]}</div>;
      }
    }
    var container = document.createElement('div');
    var ex;
    try {
      ReactDOM.render(<Foo />, container);
    } catch (e) {
      ex = e;
    }
    expect(ex).toBeDefined();
    expect(normalizeCodeLocInfo(ex.message)).toBe(
      'Objects are not valid as a React child (found: object with keys ' +
        '{a, b, c}). If you meant to render a collection of children, use ' +
        'an array instead.\n' +
        
        (ReactDOMFeatureFlags.useFiber ? '    in div (at **)\n' : '') +
        '    in Foo (at **)',
    );
  });
});