Can't easily get ALL headers for a storage object #285

I needed to get a header that isn't one of the built-in ones (ETag, Content-Length, Last-Modified, Content-Type) or metadata (ie. starting with 'X-Object-Meta-'), and there wasn't an easy way to do it.

(Specifically, I needed to get the 'X-Object-Manifest' header, because $object->delete() on a large object won't actually delete the segments that make it up, only the manifest! See #286)

I had do something like this:

class MyClass {
  protected function apiHeadObject() {
    $params = new Params();
    return [
        'method' => 'HEAD',
        'path'   => '{containerName}/{+name}',
        'params' => [
            'containerName' => $params->containerName(),
            'name'          => $params->objectName(),
        ],
    ];
  }

  public function getHeaders($container, $object_name) {
    $object = $this->service
      ->getContainer($container)
      ->getObject($object_name);

    /** @var \Psr\Http\Message\ResponseInterface $response */
    $response = $object->executeWithState($this->apiHeadObject());
    return $response->getHeaders();
  }
}

Given that there's lots of extensions to OpenStack which can be optionally enabled, including custom ones that a hosting provider may have created, it'd be nice to just have a way to arbitrarily get headers from an object.

Perhaps anything starting with 'X-' (other than metadata) could be captured in an additional array?