Explicitely set service attributes to null

[jtsaito]

This PR addresses a change in the behaviour /services POST endpoint in the V2 API of Pagerduty (see documentation). Due to the change we had issues with our Terraform setup.

Problem
When creating a service without acknowledgement_timeout or auto_resolve_timeout, the service has acknowledgement timeout feature or auto resolve timeout feature enabled respectively. This was not the case so far. There is no longer a way to create a resource with the features disabled.

Expected behaviour:
When creating a pagerduty.Service without acknowledgement_timeout or auto_resolve_timeout by calling client.CreateService(service), the service disables the features and does not set acknowledgement_timeout and auto_resolve_timeout. (This used to be the case until a few days ago.)

Observed behaviour:
The service enables the features and sets non-zero default values for acknowledgement_timeout and auto_resolve_timeout.

Example code for reproducing the issue

package main

import (
  "fmt"

  pagerduty "github.com/PagerDuty/go-pagerduty"
)

func main() {
  client := pagerduty.NewClient("YOUR-API-TOKEN-HERE")

  epo := pagerduty.GetEscalationPolicyOptions{}
  ep, epErr := client.GetEscalationPolicy("PH77HFE", &epo)

  if epErr != nil {
    fmt.Printf("error: %v\n", epErr)
  }

  s := pagerduty.Service{
    Name:               "Cha0tic App",
    Description:        "Some infinities are bigger than other infinities",
    EscalationPolicy:   *ep,
    AutoResolveTimeout: nil,
  }

  t, e := client.CreateService(s)
  if e != nil {
    fmt.Printf("error: %v\n", e)
  }

  fmt.Println(t)
}

Submitted solution
We drop omitempty for the attributes in question. This restores the default behaviour of client.CreateService(service).

[jtsaito]

Sorry, there has already been a PR for the same issue: #53. Please merge that.

Closing this one.