rgw/storage class: don't store/report STANDARD storage class.

rgw/storage class: don't store/report STANDARD storage class.

While 'STANDARD' is a valid storage class, it is not supposed
to ever be returned when fetching an object.  This change suppresses
storing 'STANDARD' as the attribute value, so that objects
explicitly created with 'STANDARD' will in fact be indistinguishable
from those where it was implicitly set.

Fixes: https://tracker.ceph.com/issues/67786
Signed-off-by: Marcus Watts <mwatts@redhat.com>
(cherry picked from commit b95e743ab9374cd3463a29c5f719ffce1c9fb28a)

Resolves: rhbz#2300284