Fix wrong retain flag when distributing messages.

Source/MQTTnet/Server/MqttClientSession.cs

@@ -227,7 +227,7 @@ namespace MQTTnet.Server
                 Topic = publishPacket.Topic,
                 Payload = publishPacket.Payload,
                 QualityOfServiceLevel = checkSubscriptionsResult.QualityOfServiceLevel,
-                Retain = false,
+                Retain = publishPacket.Retain,
                 Dup = false
             };
 
@@ -371,7 +371,12 @@ namespace MQTTnet.Server
             var retainedMessages = _retainedMessagesManager.GetSubscribedMessages(topicFilters);
             foreach (var applicationMessage in retainedMessages)
             {
-                EnqueueApplicationMessage(null, applicationMessage.ToPublishPacket());
+                var publishPacket = applicationMessage.ToPublishPacket();
+
+                // Set the retain flag to true according to [MQTT-3.3.1-8].
+                publishPacket.Retain = true;
+
+                EnqueueApplicationMessage(null, publishPacket);
             }
         }
 

Source/MQTTnet/Server/MqttClientSessionsManager.cs

@@ -185,7 +185,12 @@ namespace MQTTnet.Server
 
                 foreach (var clientSession in GetSessions())
                 {
-                    clientSession.EnqueueApplicationMessage(enqueuedApplicationMessage.Sender, applicationMessage.ToPublishPacket());
+                    var publishPacket = applicationMessage.ToPublishPacket();
+
+                    // Set the retain flag to true according to [MQTT-3.3.1-9].
+                    publishPacket.Retain = false;
+
+                    clientSession.EnqueueApplicationMessage(enqueuedApplicationMessage.Sender, publishPacket);
                 }
             }
             catch (OperationCanceledException)