include ban reasons when banning a user that already had a member event

The case where the user never had a member event in the room is already
handled correctly.

src/api/client_server/membership.rs

@@ -288,6 +288,7 @@ pub(crate) async fn ban_user_route(
                         RoomMemberEventContent {
                             membership: MembershipState::Ban,
                             join_authorized_via_users_server: None,
+                            reason: body.reason.clone(),
                             ..event
                         }
                     })