Fixed Bug in Event Rule Positioning

[stmcallister]

Changes in the last provider release caused Event Rule positioning (both in Service Event Rules and Ruleset Rules) to have issues setting position 0, as well as ignoring when positions weren't set. This fix should change that. #300

RulesetRule Test Results

TF_ACC=1 go test -run "TestAccPagerDutyRulesetRule" ./pagerduty -v -timeout 120m
=== RUN   TestAccPagerDutyRulesetRule_import
--- PASS: TestAccPagerDutyRulesetRule_import (4.86s)
=== RUN   TestAccPagerDutyRulesetRule_Basic
--- PASS: TestAccPagerDutyRulesetRule_Basic (6.06s)
=== RUN   TestAccPagerDutyRulesetRule_MultipleRules
--- PASS: TestAccPagerDutyRulesetRule_MultipleRules (6.12s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	18.277s

ServiceEventRule Test Rules

TF_ACC=1 go test -run "TestAccPagerDutyServiceEventRule" ./pagerduty -v -timeout 120m        
=== RUN   TestAccPagerDutyServiceEventRule_import
--- PASS: TestAccPagerDutyServiceEventRule_import (11.76s)
=== RUN   TestAccPagerDutyServiceEventRule_Basic
--- PASS: TestAccPagerDutyServiceEventRule_Basic (12.94s)
=== RUN   TestAccPagerDutyServiceEventRule_MultipleRules
--- PASS: TestAccPagerDutyServiceEventRule_MultipleRules (12.96s)
PASS
ok  	github.com/terraform-providers/terraform-provider-pagerduty/pagerduty	38.307s