rgw: send user manifest header field

Fixes: #8170
Backport: firefly
If user manifest header exists (swift) send it as part of the object
header data.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #1773 from ceph/wip-8170

rgw: send user manifest header field

Reviewed-by: Sage Weil <sage@inktank.com>