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(
            <ControlledTextArea onChange={() => changeCount++} />,
          );
          expect(changeCount).toBe(0);
          expect(e.value).toBe('Hello');
          
          e.value = 'Goodbye';
          ReactTestUtils.Simulate.change(e);
          expect(changeCount).toBe(1);
          expect(e.value).toBe('Goodbye');
        });
        itClientRenders('a controlled checkbox', async render => {
          let changeCount = 0;
          const e = await render(
            <ControlledCheckbox onChange={() => changeCount++} />,
          );
          expect(changeCount).toBe(0);
          expect(e.checked).toBe(true);
          
          e.checked = false;
          ReactTestUtils.Simulate.change(e);
          expect(changeCount).toBe(1);
          expect(e.checked).toBe(false);
        });
        itClientRenders('a controlled select', async render => {
          let changeCount = 0;
          const e = await render(
            <ControlledSelect onChange={() => changeCount++} />,
          );
          expect(changeCount).toBe(0);