should be no need to check if packet responds to qos

src/lavinmq/mqtt/exchange.cr

@@ -46,7 +46,7 @@ module LavinMQ
         @publish_in_count += 1
         headers = AMQP::Table.new({ RETAIN_HEADER: true }) if packet.retain?
         properties = AMQP::Properties.new(headers: headers)
-        properties.delivery_mode = packet.qos if packet.responds_to?(:qos)
+        properties.delivery_mode = packet.qos
 
         timestamp = RoughTime.unix_ms
         bodysize = packet.payload.size.to_u64