set category id on model update

src/Discord.Net.Rest/Entities/Channels/RestForumChannel.cs

@@ -85,6 +85,8 @@ namespace Discord.Rest
                 else
                     DefaultReactionEmoji = null;
             }
+
+            CategoryId = model.CategoryId.GetValueOrDefault();
         }
 
         /// <inheritdoc/>

src/Discord.Net.WebSocket/Entities/Channels/SocketForumChannel.cs

@@ -91,6 +91,8 @@ namespace Discord.WebSocket
                 else
                     DefaultReactionEmoji = null;
             }
+
+            CategoryId = model.CategoryId.GetValueOrDefault();
         }
 
         /// <inheritdoc />