toHeaderValue method
Converts to STOMP header value
Implementation
String toHeaderValue() {
  switch (this) {
    case StompAckMode.auto:
      return StompHeaders.ackAuto;
    case StompAckMode.client:
      return StompHeaders.ackClient;
    case StompAckMode.clientIndividual:
      return StompHeaders.ackClientIndividual;
  }
}