rgw: swift update obj metadata also add generic attrs

Fixes: #6462
We were missing the generic attributes when we updated the object
metadata (operation that only exists in the swift api).

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit f2645e1c6d7383a0ace3b239f4304e353249c4bb)