check newInstanceID to determine registration header

apigee_sync.go

diff --git a/apigee_sync.go b/apigee_sync.go
index 4e3d89c..449780b 100644
--- a/apigee_sync.go
+++ b/apigee_sync.go
@@ -257,7 +257,7 @@
 		req.Header.Set("status", "ONLINE")
 		req.Header.Set("plugin_details", apidPluginDetails)
 
-		if apidInfo.InstanceID == "" {
+		if newInstanceID {
 			req.Header.Set("created_at_apid", time.Now().Format(time.RFC3339))
 		} else {
 			req.Header.Set("updated_at_apid", time.Now().Format(time.RFC3339))

mock_server.go

diff --git a/mock_server.go b/mock_server.go
index 982ad37..7fc488e 100644
--- a/mock_server.go
+++ b/mock_server.go
@@ -240,11 +240,9 @@
 	Expect(req.Header.Get("apid_cluster_Id")).To(Equal(m.params.ClusterID))
 	Expect(req.Header.Get("display_name")).ToNot(BeEmpty())
 
-	if req.Header.Get("apid_instance_id") == "" {
-		Expect(req.Header.Get("created_at_apid")).ToNot(BeEmpty())
+	if req.Header.Get("created_at_apid") != "" {
 		Expect(req.Header.Get("updated_at_apid")).To(BeEmpty())
 	} else {
-		Expect(req.Header.Get("created_at_apid")).To(BeEmpty())
 		Expect(req.Header.Get("updated_at_apid")).ToNot(BeEmpty())
 	}