Fix memory leak

[attilakunelwood]

This commit fixes a memory leak that happens if the PagerDuty endpoint returns a 429 HTTP status code. It was not verified whether the error happens with other status codes as well.

I verified the correctness of this fix by running the following test:

func TestMemoryLeak(t *testing.T) {
	evt := &pagerduty.V2Event{
		RoutingKey: routingKey,
		Action:     "trigger",
		Payload: &pagerduty.V2Payload{
			Summary:   "foo",
			Source:    "bar",
			Component: serviceName,
			Severity:  "critical",
			Class:     "",
			Details:   nil,
		},
	}

	for i := 0; i < 100; i++ {
		client.ManageEvent(evt)
		fmt.Printf("num goroutines: %d\n", runtime.NumGoroutine())
		time.Sleep(time.Millisecond * 50)
	}
}

Without the fix, this produces the following output:

num goroutines: 6
num goroutines: 8
num goroutines: 10
num goroutines: 12
num goroutines: 14
num goroutines: 16
num goroutines: 18
num goroutines: 20
num goroutines: 22
num goroutines: 24
...

With the fix, this produces the following output:

num goroutines: 8
num goroutines: 8
num goroutines: 8
num goroutines: 8
num goroutines: 8
num goroutines: 8
num goroutines: 6
num goroutines: 6
num goroutines: 6
num goroutines: 6
num goroutines: 5
num goroutines: 6
...

[acloudgopher-pd]

Thank you for the fix!

[ChuckCrawford]

Yeah thank you for the fix @attilakunelwood! We are going to have one more look at this and see if there isn't a more centralized location for us to put this fix.