Fix NotificationMailers::Base#log_mail with frozen strings

app/mailers/notification_mailers/base.rb

@@ -54,7 +54,7 @@ module NotificationMailers
     def log_mail(recipient_id, sender_id, type)
       log_string = "event=mail mail_type=#{type} recipient_id=#{recipient_id} sender_id=#{sender_id} " \
                    " recipient_handle=#{@recipient.diaspora_handle}"
-      log_string << " sender_handle=#{@sender.diaspora_handle}" if sender_id.present?
+      log_string = "#{log_string} sender_handle=#{@sender.diaspora_handle}" if sender_id.present?
 
       logger.info log_string
     end