add 204 as a possible cors preflight response code (#4637)

A CORS preflight response has only headers, and some implementations use
204 No Content as the response code. This update adds 204 along with 200
as valid response codes for preflight requests.

Change also regular (non preflight) responses to status code 200 only,
since this is a response of a test backend configured by the conformance
test.

Co-authored-by: Joao Morais <jcmoraisjr@gmail.com>

Author: k8s-infra-cherrypick-robot

conformance/tests/httproute-cors.go

@@ -127,7 +127,7 @@ var HTTPRouteCORS = suite.ConformanceTest{
 				},
 				Namespace: "",
 				Response: http.Response{
-					StatusCode: 200,
+					StatusCodes: []int{200, 204},
 					ValidHeaderValues: map[string][]string{
 						"access-control-allow-origin": {"https://www.bar.com"},
 						"access-control-allow-methods": {
@@ -175,7 +175,7 @@ var HTTPRouteCORS = suite.ConformanceTest{
 				},
 				Namespace: "",
 				Response: http.Response{
-					StatusCode: 200,
+					StatusCodes: []int{200, 204},
 					ValidHeaderValues: map[string][]string{
 						"access-control-allow-origin": {"https://xpto.www.bar.com"},
 						"access-control-allow-methods": {
@@ -241,7 +241,7 @@ var HTTPRouteCORS = suite.ConformanceTest{
 					},
 				},
 				Response: http.Response{
-					StatusCodes: []int{200, 204},
+					StatusCode: 200,
 					Headers: map[string]string{
 						"access-control-allow-origin": "https://www.foo.com",
 					},
@@ -260,7 +260,7 @@ var HTTPRouteCORS = suite.ConformanceTest{
 					},
 				},
 				Response: http.Response{
-					StatusCodes: []int{200, 204},
+					StatusCode: 200,
 					Headers: map[string]string{
 						"access-control-allow-origin": "https://www.bar.com",
 					},
@@ -341,6 +341,7 @@ var HTTPRouteCORS = suite.ConformanceTest{
 				},
 				Namespace: "",
 				Response: http.Response{
+					StatusCodes:   []int{200, 204},
 					AbsentHeaders: []string{"Access-Control-Allow-Credentials"},
 				},
 			},
@@ -424,7 +425,7 @@ var HTTPRouteCORS = suite.ConformanceTest{
 					},
 				},
 				Response: http.Response{
-					StatusCodes: []int{200},
+					StatusCode: 200,
 					ValidHeaderValues: map[string][]string{
 						// The access-control-allow-origin for a wildcard domain depends on the implementation.
 						// Envoy enforces the return of the same requested Origin, while NGINX an others may return a "*"
@@ -448,7 +449,7 @@ var HTTPRouteCORS = suite.ConformanceTest{
 					},
 				},
 				Response: http.Response{
-					StatusCodes: []int{200, 204},
+					StatusCode: 200,
 					ValidHeaderValues: map[string][]string{
 						// The access-control-allow-origin for a wildcard domain depends on the implementation.
 						// Envoy enforces the return of the same requested Origin, while NGINX an others may return a "*"
@@ -488,7 +489,7 @@ var HTTPRouteCORS = suite.ConformanceTest{
 				},
 				Namespace: "",
 				Response: http.Response{
-					StatusCode: 200,
+					StatusCodes: []int{200, 204},
 					ValidHeaderValues: map[string][]string{
 						"access-control-allow-origin":  {"https://other.foo.com"},
 						"access-control-allow-methods": {"PUT"},
@@ -528,7 +529,7 @@ var HTTPRouteCORS = suite.ConformanceTest{
 				},
 				Namespace: "",
 				Response: http.Response{
-					StatusCode: 200,
+					StatusCodes: []int{200, 204},
 					ValidHeaderValues: map[string][]string{
 						"access-control-allow-origin": {"https://other.foo.com", "*"},
 						"access-control-allow-methods": {