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**
const e = await render(<select value="bar">{options}</select>, 1);
          expectSelectValue(e, 'bar');
        },
      );
      itRenders('a select with a defaultValue', async render => {
        const e = await render(<select defaultValue="bar">{options}</select>);
        expectSelectValue(e, 'bar');
      });
      itRenders('a select value overriding defaultValue', async render => {
        const e = await render(
          <select value="bar" defaultValue="baz" readOnly={true}>
            {options}
          </select>,
          1,
        );
        expectSelectValue(e, 'bar');
      });
      itRenders(
        'a select value overriding defaultValue no matter the prop order',
        async render => {
          const e = await render(
            <select defaultValue="baz" value="bar" readOnly={true}>
              {options}
            </select>,
            1,
          );
          expectSelectValue(e, 'bar');
        },