def ack(delivery_tag, multiple = false)
      if multiple
        @acknowledged_state[:pending].keys.each do |key|
          ack(key, false) if key <= delivery_tag
        end
      elsif @acknowledged_state[:pending].key?(delivery_tag)
        update_acknowledgement_state(delivery_tag, :acked)
      end
      nil
    end
def ack(delivery_tag, multiple = false)
      if multiple
        @acknowledged_state[:pending].keys.each do |key|
          ack(key, false) if key <= delivery_tag
        end
      elsif @acknowledged_state[:pending].key?(delivery_tag)
        update_acknowledgement_state(delivery_tag, :acked)
      end
      nil
    end
Acknowledge message.

 @param [Integer] delivery_tag Delivery tag to acknowledge
 @param [Boolean] multiple (false) Should all unacknowledged messages up to this be acknowleded as well?

 @return nil
 @api public