Fix packet identifier which is sent from the server.

Frameworks/MQTTnet.NetStandard/Packets/IMqttPacketWithIdentifier.cs

@@ -2,6 +2,6 @@
 {
     public interface IMqttPacketWithIdentifier
     {
-        ushort PacketIdentifier { get; set; }
+        ushort? PacketIdentifier { get; set; }
     }
 }

Frameworks/MQTTnet.NetStandard/Packets/MqttBasePublishPacket.cs

@@ -2,6 +2,6 @@
 {
     public class MqttBasePublishPacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
-        public ushort PacketIdentifier { get; set; }
+        public ushort? PacketIdentifier { get; set; }
     }
 }

Frameworks/MQTTnet.NetStandard/Packets/MqttPacketExtensions.cs

@@ -1,26 +0,0 @@
-using System;
-
-namespace MQTTnet.Packets
-{
-    public static class MqttPacketExtensions
-    {
-        public static TResponsePacket CreateResponse<TResponsePacket>(this MqttBasePacket packet)
-        {
-            if (packet == null) throw new ArgumentNullException(nameof(packet));
-
-            var responsePacket = Activator.CreateInstance<TResponsePacket>();
-
-            if (responsePacket is IMqttPacketWithIdentifier responsePacketWithIdentifier)
-            {
-                if (!(packet is IMqttPacketWithIdentifier requestPacketWithIdentifier))
-                {
-                    throw new InvalidOperationException("Response packet has PacketIdentifier but request packet does not.");
-                }
-
-                responsePacketWithIdentifier.PacketIdentifier = requestPacketWithIdentifier.PacketIdentifier;
-            }
-
-            return responsePacket;
-        }
-    }
-}

Frameworks/MQTTnet.NetStandard/Packets/MqttSubAckPacket.cs

@@ -6,7 +6,7 @@ namespace MQTTnet.Packets
 {
     public sealed class MqttSubAckPacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
-        public ushort PacketIdentifier { get; set; }
+        public ushort? PacketIdentifier { get; set; }
 
         public IList<MqttSubscribeReturnCode> SubscribeReturnCodes { get; } = new List<MqttSubscribeReturnCode>();
 

Frameworks/MQTTnet.NetStandard/Packets/MqttSubscribePacket.cs

@@ -5,7 +5,7 @@ namespace MQTTnet.Packets
 {
     public sealed class MqttSubscribePacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
-        public ushort PacketIdentifier { get; set; }
+        public ushort? PacketIdentifier { get; set; }
 
         public IList<TopicFilter> TopicFilters { get; set; } = new List<TopicFilter>();
 

Frameworks/MQTTnet.NetStandard/Packets/MqttUnsubAckPacket.cs

@@ -2,7 +2,7 @@
 {
     public sealed class MqttUnsubAckPacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
-        public ushort PacketIdentifier { get; set; }
+        public ushort? PacketIdentifier { get; set; }
 
         public override string ToString()
         {

Frameworks/MQTTnet.NetStandard/Packets/MqttUnsubscribe.cs

@@ -4,7 +4,7 @@ namespace MQTTnet.Packets
 {
     public sealed class MqttUnsubscribePacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
-        public ushort PacketIdentifier { get; set; }
+        public ushort? PacketIdentifier { get; set; }
 
         public IList<string> TopicFilters { get; set; } = new List<string>();