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**
options = [
          <option key={1} value="foo" id="foo">Foo</option>,
          <option key={2} value="bar" id="bar">Bar</option>,
          <option key={3} value="baz" id="baz">Baz</option>,
        ];
      });
      
      
      
      const expectSelectValue = (element, selected) => {
        if (!Array.isArray(selected)) {
          selected = [selected];
        }
        
        
        expect(element.getAttribute('value')).toBe(null);
        expect(element.getAttribute('defaultValue')).toBe(null);
        ['foo', 'bar', 'baz'].forEach(value => {
          const expectedValue = selected.indexOf(value) !== -1;
          var option = element.querySelector(`#${value}`);
          expect(option.selected).toBe(expectedValue);
        });
      };
      itRenders('a select with a value and an onChange', async render => {
        const e = await render(
          <select value="bar" onChange={() => {}}>{options}</select>,
        );
        expectSelectValue(e, 'bar');
      });
      itRenders('a select with a value and readOnly', async render => {