Merge pull request #326 from hchen2020/master

Fix user id.

Author: Oceania2018

src/Infrastructure/BotSharp.Core/Conversations/Services/ConversationService.SendMessage.cs

@@ -27,7 +27,10 @@ public async Task<bool> SendMessage(string agentId,
 #endif
 
         message.CurrentAgentId = agent.Id;
-        message.SenderId = _user.Id;
+        if (string.IsNullOrEmpty(message.SenderId))
+        {
+            message.SenderId = _user.Id;
+        }
 
         _storage.Append(_conversationId, message);