Add missing union specification for MatchUserRequest

osu.Game/Online/Multiplayer/MatchUserRequest.cs

@@ -3,6 +3,7 @@
 
 using System;
 using MessagePack;
+using osu.Game.Online.Multiplayer.MatchTypes.TeamVersus;
 
 namespace osu.Game.Online.Multiplayer
 {
@@ -11,6 +12,7 @@ namespace osu.Game.Online.Multiplayer
     /// </summary>
     [Serializable]
     [MessagePackObject]
+    [Union(0, typeof(ChangeTeamRequest))] // IMPORTANT: Add rules to SignalRUnionWorkaroundResolver for new derived types.
     public abstract class MatchUserRequest
     {
     }