Update docs. Add UnitTests.

Source/MQTTnet.Server/Mqtt/MqttApplicationMessageInterceptor.cs

@@ -22,7 +22,8 @@ namespace MQTTnet.Server.Mqtt
         {
             try
             {
-                var sessionItems = (PythonDictionary)context.SessionItems[MqttServerConnectionValidator.WrappedSessionItemsKey];
+                // This might be not set when a message was published by the server instead of a client.
+                context.SessionItems.TryGetValue(MqttServerConnectionValidator.WrappedSessionItemsKey, out var sessionItems);
 
                 var pythonContext = new PythonDictionary
                 {
@@ -50,4 +51,4 @@ namespace MQTTnet.Server.Mqtt
             return Task.CompletedTask;
         }
     }
-}
+}

Source/MQTTnet/Packets/MqttUserProperty.cs

@@ -13,5 +13,31 @@ namespace MQTTnet.Packets
         public string Name { get; }
 
         public string Value { get; }
+
+        public override int GetHashCode()
+        {
+            return Name.GetHashCode() ^ Value.GetHashCode();
+        }
+
+        public override bool Equals(object other)
+        {
+            return Equals(other as MqttUserProperty);
+        }
+
+        public bool Equals(MqttUserProperty other)
+        {
+            if (other == null)
+            {
+                return false;
+            }
+
+            if (ReferenceEquals(other, this))
+            {
+                return true;
+            }
+
+            return string.Equals(Name, other.Name, StringComparison.Ordinal) &&
+                string.Equals(Value, other.Value, StringComparison.Ordinal);
+        }
     }
 }