Fix context menu comand message type (#2128)

src/Discord.Net.WebSocket/Entities/Messages/SocketMessage.cs

@@ -134,7 +134,8 @@ namespace Discord.WebSocket
             if (model.Type == MessageType.Default ||
                 model.Type == MessageType.Reply ||
                 model.Type == MessageType.ApplicationCommand ||
-                model.Type == MessageType.ThreadStarterMessage)
+                model.Type == MessageType.ThreadStarterMessage ||
+                model.Type == MessageType.ContextMenuCommand)
                 return SocketUserMessage.Create(discord, state, author, channel, model);
             else
                 return SocketSystemMessage.Create(discord, state, author, channel, model);