frontend: Fix status code for subscription PUT

By my read of the Subscription Lifecycle API Reference [1], we
should favor 200 OK over 201 Created when creating or updating
a subscription.

[1]
https://github.com/cloud-and-ai-microsoft/resource-provider-contract/blob/master/v1.0/subscription-lifecycle-api-reference.md#response

frontend/pkg/frontend/frontend.go

@@ -726,7 +726,7 @@ func (f *Frontend) ArmSubscriptionPut(writer http.ResponseWriter, request *http.
 		"state":          string(subscription.State),
 	})
 
-	_, err = arm.WriteJSONResponse(writer, http.StatusCreated, subscription)
+	_, err = arm.WriteJSONResponse(writer, http.StatusOK, subscription)
 	if err != nil {
 		f.logger.Error(err.Error())
 	}

frontend/pkg/frontend/frontend_test.go

@@ -139,7 +139,7 @@ func TestSubscriptionsPUT(t *testing.T) {
 				Properties:       nil,
 			},
 			subDoc:             nil,
-			expectedStatusCode: http.StatusCreated,
+			expectedStatusCode: http.StatusOK,
 		},
 		{
 			name:    "PUT Subscription - Doc Exists",
@@ -159,7 +159,7 @@ func TestSubscriptionsPUT(t *testing.T) {
 					Properties:       nil,
 				},
 			},
-			expectedStatusCode: http.StatusCreated,
+			expectedStatusCode: http.StatusOK,
 		},
 		{
 			name:    "PUT Subscription - Invalid Subscription",