rgw: add xml output header in RGWCopyObj_ObjStore_S3 response msg

Fixes: http://tracker.ceph.com/issues/22416

Signed-off-by: Enming Zhang <enming.zhang@umcloud.com>
(cherry picked from commit 381a5b5)

src/rgw/rgw_rest_s3.cc

@@ -2128,6 +2128,7 @@ void RGWCopyObj_ObjStore_S3::send_partial_response(off_t ofs)
     dump_errno(s);
 
     end_header(s, this, "application/xml");
+    dump_start(s);
     if (op_ret == 0) {
       s->formatter->open_object_section_in_ns("CopyObjectResult", XMLNS_AWS_S3);
     }