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**
);
    });
    describe('textareas', function() {
      
      
      itRenders('a textarea with a value and an onChange', async render => {
        const e = await render(<textarea value="foo" onChange={() => {}} />);
        
        
        expect(e.getAttribute('value')).toBe(null);
        expect(e.value).toBe('foo');
      });
      itRenders('a textarea with a value and readOnly', async render => {
        const e = await render(<textarea value="foo" readOnly={true} />);
        
        
        expect(e.getAttribute('value')).toBe(null);
        expect(e.value).toBe('foo');
      });
      itRenders(
        'a textarea with a value and no onChange/readOnly',
        async render => {
          
          
          const e = await render(<textarea value="foo" />, 1);
          expect(e.getAttribute('value')).toBe(null);
          expect(e.value).toBe('foo');
        },
      );
      itRenders('a textarea with a defaultValue', async render => {