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**
'value if `multiple` is false.%s',
              propName,
              '', 
            );
          }
        }
        if (
          props.value !== undefined &&
          props.defaultValue !== undefined &&
          !didWarnDefaultSelectValue
        ) {
          warning(
            false,
            'Select elements must be either controlled or uncontrolled ' +
              '(specify either the value prop, or the defaultValue prop, but not ' +
              'both). Decide between using a controlled or uncontrolled select ' +
              'element and remove one of these props. More info: ' +
              'https://fb.me/react-controlled-components',
          );
          didWarnDefaultSelectValue = true;
        }
      }
      this.currentSelectValue = props.value != null
        ? props.value
        : props.defaultValue;
      props = Object.assign({}, props, {
        value: undefined,
      });
    } else if (tag === 'option') {
      var selected = null;