fix: false-positive detection of CustomStatusGame based on Id property

This change fixes a bug that was introduced in PR #1406. Games were falsely detected to be CustomStatusGames, based on the Id property being included in the model payload.

This fixes the false detection of Games as CustomStatusGame. An activity will only be considered a CustomStatusGame if the Id has a value of "custom".

This change has been tested by listening to the GuildMemberUpdated event, and opening/closing games with a custom status set.

src/Discord.Net.WebSocket/Extensions/EntityExtensions.cs

@@ -10,7 +10,7 @@ namespace Discord.WebSocket
         public static IActivity ToEntity(this API.Game model)
         {
             // Custom Status Game
-            if (model.Id.IsSpecified)
+            if (model.Id.IsSpecified && model.Id.Value == "custom")
             {
                 return new CustomStatusGame()
                 {