rgw: fix cast

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: add some comments

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
PendingReleaseNotes: update about new rgw copy obj response

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>