chat: return username in Recipient for channels

chat.go

@@ -65,6 +65,9 @@ type ChatPhoto struct {
 
 // Recipient returns chat ID (see Recipient interface).
 func (c *Chat) Recipient() string {
+	if c.Type == ChatChannel && c.Username != "" {
+		return c.Username
+	}
 	return strconv.FormatInt(c.ID, 10)
 }