Fix upgrading and downgrading of retained messages QoS for subscriptions (#1564)

Source/MQTTnet/Formatter/MqttPacketFactories.cs

@@ -7,6 +7,7 @@ namespace MQTTnet.Formatter
     public sealed class MqttPacketFactories
     {
         public MqttConnAckPacketFactory ConnAck { get; } = new MqttConnAckPacketFactory();
+        
         public MqttConnectPacketFactory Connect { get; } = new MqttConnectPacketFactory();
 
         public MqttDisconnectPacketFactory Disconnect { get; } = new MqttDisconnectPacketFactory();

Source/MQTTnet/Formatter/MqttPublishPacketFactory.cs

@@ -5,6 +5,7 @@
 using System;
 using MQTTnet.Exceptions;
 using MQTTnet.Packets;
+using MQTTnet.Server;
 
 namespace MQTTnet.Formatter
 {
@@ -80,5 +81,17 @@ namespace MQTTnet.Formatter
 
             return packet;
         }
+
+        public MqttPublishPacket Create(MqttRetainedMessageMatch retainedMessage)
+        {
+            if (retainedMessage == null)
+            {
+                throw new ArgumentNullException(nameof(retainedMessage));
+            }
+
+            var publishPacket = Create(retainedMessage.ApplicationMessage);
+            publishPacket.QualityOfServiceLevel = retainedMessage.SubscriptionQualityOfServiceLevel;
+            return publishPacket;
+        }
     }
 }