rgw: swift GET / HEAD object returns X-Timestamp field

Fixes: #8911
Backport: giant, firefly, dumpling
Swift clients expect X-Timestamp header, dump it.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #2624 from ceph/wip-8911

rgw: swift GET / HEAD object returns X-Timestamp field

Reviewed-by: Sage Weil <sage@redhat.com>