Actually populate SocketSimpleUser's fields

src/Discord.Net.WebSocket/Entities/Users/SocketSimpleUser.cs

@@ -29,6 +29,14 @@ namespace Discord.WebSocket
 
         internal override void Update(ClientState state, PresenceModel model)
         {
+            if (model.User.Avatar.IsSpecified)
+                AvatarId = model.User.Avatar.Value;
+            if (model.User.Discriminator.IsSpecified)
+                DiscriminatorValue = ushort.Parse(model.User.Discriminator.Value);
+            if (model.User.Bot.IsSpecified)
+                IsBot = model.User.Bot.Value;
+            if (model.User.Username.IsSpecified)
+                Username = model.User.Username.Value;
         }
 
         internal new SocketSimpleUser Clone() => MemberwiseClone() as SocketSimpleUser;