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" readOnly={true}>{options}</select>,
        );
        expectSelectValue(e, 'bar');
      });
      itRenders(
        'a select with a multiple values and an onChange',
        async render => {
          const e = await render(
            <select value={['bar', 'baz']} multiple={true} onChange={() => {}}>
              {options}
            </select>,
          );
          expectSelectValue(e, ['bar', 'baz']);
        },
      );
      itRenders(
        'a select with a multiple values and readOnly',
        async render => {
          const e = await render(
            <select value={['bar', 'baz']} multiple={true} readOnly={true}>
              {options}
            </select>,
          );
          expectSelectValue(e, ['bar', 'baz']);
        },
      );
      itRenders(
        'a select with a value and no onChange/readOnly',
        async render => {