Use 200 OK in responses for preflight requests

Some browsers only accept 200 OK and not 204 No Content as respones codes
for preflight requests.

See tus/tus-js-client#34

concat_test.go

@@ -48,7 +48,7 @@ func TestConcatPartial(t *testing.T) {
 		ResHeader: map[string]string{
 			"Tus-Extension": "creation,concatenation",
 		},
-		Code: http.StatusNoContent,
+		Code: http.StatusOK,
 	}).Run(handler, t)
 
 	(&httpTest{

cors_test.go

@@ -20,7 +20,7 @@ func TestCORS(t *testing.T) {
 		ReqHeader: map[string]string{
 			"Origin": "tus.io",
 		},
-		Code: http.StatusNoContent,
+		Code: http.StatusOK,
 		ResHeader: map[string]string{
 			"Access-Control-Allow-Headers": "",
 			"Access-Control-Allow-Methods": "",

options_test.go

@@ -18,7 +18,7 @@ func TestOptions(t *testing.T) {
 	(&httpTest{
 		Name:   "Successful request",
 		Method: "OPTIONS",
-		Code:   http.StatusNoContent,
+		Code:   http.StatusOK,
 		ResHeader: map[string]string{
 			"Tus-Extension": "creation",
 			"Tus-Version":   "1.0.0",

terminate_test.go

@@ -46,7 +46,7 @@ func TestTerminate(t *testing.T) {
 		ResHeader: map[string]string{
 			"Tus-Extension": "creation,termination",
 		},
-		Code: http.StatusNoContent,
+		Code: http.StatusOK,
 	}).Run(handler, t)
 
 	(&httpTest{

unrouted_handler.go

@@ -158,7 +158,14 @@ func (handler *UnroutedHandler) Middleware(h http.Handler) http.Handler {
 			header.Set("Tus-Version", "1.0.0")
 			header.Set("Tus-Extension", handler.extensions)
 
-			w.WriteHeader(http.StatusNoContent)
+			// Although the 204 No Content status code is a better fit in this case,
+			// since we do not have a response body included, we cannot use it here
+			// as some browsers only accept 200 OK as successful response to a
+			// preflight request. If we send them the 204 No Content the response
+			// will be ignored or interpreted as a rejection.
+			// For example, the Presto engine, which is used in older versions of
+			// Opera, Opera Mobile and Opera Mini, handles CORS this way.
+			w.WriteHeader(http.StatusOK)
 			return
 		}