Revert "Removed Message.Acknowledge"

This reverts commit eb34c6a6b2.

src/Discord.Net/Legacy.cs

@@ -169,8 +169,12 @@ namespace Discord.Legacy
             return channel.DownloadMessages(limit, relativeMessageId, relativeDir, useCache);
         }
 
-        [Obsolete("Removed in 0.9a3", true)]
-        public static Task AckMessage(this DiscordClient client, Message message) => null;
+        [Obsolete("Use Message.Acknowledge")]
+        public static Task AckMessage(this DiscordClient client, Message message)
+        {
+            if (message == null) throw new ArgumentNullException(nameof(message));
+            return message.Acknowledge();
+        }
 
         /*[Obsolete("Use Channel.ImportMessages")]
         public IEnumerable<Message> ImportMessages(Channel channel, string json)

src/Discord.Net/Models/Message.cs

@@ -337,6 +337,14 @@ namespace Discord
                 catch (HttpException ex) when (ex.StatusCode == HttpStatusCode.NotFound) { }
             }
         }
+        
+        public Task Acknowledge()
+        {
+            if (User.Id != Client.CurrentUser.Id)
+                return Client.ClientAPI.Send(new AckMessageRequest(Channel.Id, Id));
+            else
+                return TaskHelper.CompletedTask;
+        }
 
         /// <summary> Returns true if the logged-in user was mentioned. </summary>
         public bool IsMentioningMe(bool includeRoles = false)