Fix application command and thread starter messages being created as SocketSystemMessage

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

@@ -122,7 +122,10 @@ namespace Discord.WebSocket
         }
         internal static SocketMessage Create(DiscordSocketClient discord, ClientState state, SocketUser author, ISocketMessageChannel channel, Model model)
         {
-            if (model.Type == MessageType.Default || model.Type == MessageType.Reply)
+            if (model.Type == MessageType.Default ||
+                model.Type == MessageType.Reply ||
+                model.Type == MessageType.ApplicationCommand ||
+                model.Type == MessageType.ThreadStarterMessage)
                 return SocketUserMessage.Create(discord, state, author, channel, model);
             else
                 return SocketSystemMessage.Create(discord, state, author, channel, model);