Fix NotificationMailers::Base#log_mail with frozen strings

    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