Function encode_unsubscribe 

fn encode_unsubscribe<S, B>(
    session: &S,
    out: &mut PacketWriter<B>,
    packet_id: PacketId,
    max_packet_size: Option<u32>,
) -> Encoded
where
    S: SessionState,
    B: ByteBuffer,

Encodes one pending UNSUBSCRIBE group; see encode_subscribe.