rgw: copy object should not copy source acls

Fixes: #3802
Backport: argonaut, bobtail

When using the S3 api and x-amz-metadata-directive is
set to COPY we used to copy complete metadata of source
object. However, this shouldn't include the source ACLs.

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