Set message Id after queued send succeeds

src/Discord.Net/MessageQueue.cs

@@ -136,6 +136,7 @@ namespace Discord.Net
                         IsTTS = msg.IsTTS
                     };
                     var response = await _client.ClientAPI.Send(request).ConfigureAwait(false);
+                    msg.Id = response.Id;
                     msg.Update(response);
                     msg.State = MessageState.Normal;
                 }

src/Discord.Net/Models/Message.cs

@@ -171,7 +171,7 @@ namespace Discord
         internal DiscordClient Client => Channel.Client;
 
         /// <summary> Returns the unique identifier for this message. </summary>
-        public ulong Id { get; }
+        public ulong Id { get; internal set; }
         /// <summary> Returns the channel this message was sent to. </summary>
         public Channel Channel { get; }
         /// <summary> Returns the author of this message. </summary>