code: func (ar AggregatedSendResult) String() string {
  	errMap := map[string]int{}
  	for _, ack := range ar {
  		if ack.error == nil {
  			continue
  		}
  		errMap[ack.Error()]++
  	}
  
  	ackCount := ar.AckCount()
  	output := map[string]interface{}{}
  	if ackCount > 0 {
  		output["successes"] = ackCount
  	}
  	if ackCount < len(ar) {
  		output["failures"] = errMap
  	}
  	b, _ := json.Marshal(output)
  	return string(b)
  }
docs: // String returns a JSONed string representation
  // of the AggregatedSendResult