set_acl(): return 501 Not Implemented for DreamHost DreamObjects.

DreamHost doesn't implement get_acl() and set_acl() as one would
expect.  Getting the ACL with get_acl(), then setting it with
set_acl() results in an unreachable object.

For now, we will special-case the set_acl() call and prevent it from
being issued to DreamObjects, to avoid this.