rgw: S3 object copy content type fix

Fixes: #9478
Backport: firefly, giant
Content type for S3 object copy response should be set to
application/xml.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #2864 from ceph/wip-9478

rgw: S3 object copy content type fix

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>