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**
invariant(
    !Array.isArray(child),
    'The server renderer does not implement support for array children yet.',
  );
  while (React.isValidElement(child)) {
    if (__DEV__) {
      pushElementToDebugStack(child);
    }
    var Component = child.type;
    if (typeof Component !== 'function') {
      break;
    }
    var publicContext = processContext(Component, context);
    var inst;
    var queue = [];
    var replace = false;
    var updater = {
      isMounted: function(publicInstance) {
        return false;
      },
      enqueueForceUpdate: function(publicInstance) {
        if (queue === null) {
          warnNoop(publicInstance, 'forceUpdate');
          return null;
        }
      },
      enqueueReplaceState: function(publicInstance, completeState) {
        replace = true;
        queue = [completeState];