Gateway: add support for HTTP OPTIONS request type

OPTIONS is a noop request that is used by the browsers
to check if server will accept cross-site XMLHttpRequest
(indicated by the presence of CORS headers)

Before this fix user could enable CORS headers in the Gateway config,
but XHR failed due to the lack of support for OPTIONS request type
(as described in https://git.io/vzgGe)

License: MIT
Signed-off-by: Marcin Rataj <lidel@lidel.org>

core/corehttp/gateway_handler.go

@@ -82,6 +82,11 @@ func (i *gatewayHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if r.Method == "OPTIONS" {
+		i.optionsHandler(w, r)
+		return
+	}
+
 	errmsg := "Method " + r.Method + " not allowed: "
 	if !i.config.Writable {
 		w.WriteHeader(http.StatusMethodNotAllowed)
@@ -94,6 +99,15 @@ func (i *gatewayHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	log.Error(errmsg) // TODO(cryptix): log errors until we have a better way to expose these (counter metrics maybe)
 }
 
+func (i *gatewayHandler) optionsHandler(w http.ResponseWriter, r *http.Request) {
+	/*
+		OPTIONS is a noop request that is used by the browsers to check
+		if server accepts cross-site XMLHttpRequest (indicated by the presence of CORS headers)
+		https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS#Preflighted_requests
+	*/
+	i.addUserHeaders(w) // return all custom headers (including CORS ones, if set)
+}
+
 func (i *gatewayHandler) getOrHeadHandler(w http.ResponseWriter, r *http.Request) {
 	ctx, cancel := context.WithTimeout(i.node.Context(), time.Hour)
 	// the hour is a hard fallback, we don't expect it to happen, but just in case