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 clientRenderOnBadMarkup = async (element, errorCount = 0) => {
  
  var domElement = document.createElement('div');
  domElement.innerHTML =
    '<div id="badIdWhichWillCauseMismatch" data-reactroot="" data-reactid="1"></div>';
  await renderIntoDom(element, domElement, errorCount + 1);
  
  var hydratedTextContent = domElement.textContent;
  
  const cleanDomElement = document.createElement('div');
  ExecutionEnvironment.canUseDOM = true;
  await asyncReactDOMRender(element, cleanDomElement);
  ExecutionEnvironment.canUseDOM = false;
  
  const cleanTextContent = cleanDomElement.textContent;
  
  expect(hydratedTextContent).toBe(cleanTextContent);
  
  throw new BadMarkupExpected();
};
function itRenders(desc, testFn) {
  it(`renders ${desc} with server string render`, () => testFn(serverRender));
  if (ReactDOMFeatureFlags.useFiber) {
    it(`renders ${desc} with server stream render`, () => testFn(streamRender));
  }
  itClientRenders(desc, testFn);
}
function itClientRenders(desc, testFn) {