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**
itThrows(`when rendering ${desc} with server string render`, () =>
    testFn(serverRender),
  );
  itThrows(`when rendering ${desc} with clean client render`, () =>
    testFn(clientCleanRender),
  );
  
  
  itThrows(
    `when rendering ${desc} with client render on top of bad server markup`,
    () =>
      testFn((element, warningCount = 0) =>
        clientRenderOnBadMarkup(element, warningCount - 1),
      ),
  );
}
async function testMarkupMatch(serverElement, clientElement, shouldMatch) {
  const domElement = await serverRender(serverElement);
  resetModules();
  return renderIntoDom(
    clientElement,
    domElement.parentNode,
    shouldMatch ? 0 : 1,
  );
}
function expectMarkupMatch(serverElement, clientElement) {