Add support for multipart responses. Work in progress.

It's cleaner if we return `ResourceMultipartResponse` only when we have multiple parts. `$existing` will only be an array if the request is done with `&open_revs`, see the [DocIdConverter](https://github.com/dickolsson/drupal-relaxed/blob/8.x-1.x/src/ParamConverter/DocIdConverter.php#L79). So we can do something like this:

```php
if (is_array($existing)) {
  foreach ($revisions as $revision) {
    $parts[] = new ResourceResponse(
      $revision,
      // etc...
    );
  }
  return new ResourceMultipartResponse($parts, 200, array('Content-Type' => 'multipart/mixed'));
}
return new ResourceMultipartResponse($revisions[0], 200);
```