fix: payload for sendmessage to prevent upsert user

message.go

@@ -31,6 +31,7 @@ type Message struct {
 	Silent bool        `json:"silent,omitempty"`
 
 	User            *User          `json:"user"`
+	UserID          string         `json:"user_id"`
 	Attachments     []*Attachment  `json:"attachments"`
 	LatestReactions []*Reaction    `json:"latest_reactions"` // last reactions
 	OwnReactions    []*Reaction    `json:"own_reactions"`
@@ -256,7 +257,8 @@ func (ch *Channel) SendMessage(ctx context.Context, message *Message, userID str
 		return nil, errors.New("user ID must be not empty")
 	}
 
-	message.User = &User{ID: userID}
+	message.User = nil
+	message.UserID = userID
 	p := path.Join("channels", url.PathEscape(ch.Type), url.PathEscape(ch.ID), "message")
 
 	req := message.toRequest()