Address: Using pagerduty_alert_grouping_setting causes error HTTP response failed with status code 404 and no JSON error object was present

[imjaroiswebdev]

Close #944

New test case introduce to protect from regressions...

TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -count=1 -run TestAccPagerDutyAlertGroupingSetting_serviceNotExist -timeout 120m
?       github.com/PagerDuty/terraform-provider-pagerduty       [no test files]
?       github.com/PagerDuty/terraform-provider-pagerduty/util/apiutil  [no test files]
testing: warning: no tests to run
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerduty     0.569s [no tests to run]
=== RUN   TestAccPagerDutyAlertGroupingSetting_serviceNotExist
--- PASS: TestAccPagerDutyAlertGroupingSetting_serviceNotExist (16.57s)
PASS
ok      github.com/PagerDuty/terraform-provider-pagerduty/pagerdutyplugin       17.731s