Implement RFC 9110, generate less server code (#34)

* Prepare changes in generated code

* Remove one server method generated per each RPC method

* Implement RFC 9110: Respond with Allow header on HTTP 405

* Switch case on request path to choose the JSON handler

server.go.tmpl

@@ -32,41 +32,39 @@ func (s *{{$serviceName}}) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	ctx = context.WithValue(ctx, HTTPRequestCtxKey, r)
 	ctx = context.WithValue(ctx, ServiceNameCtxKey, "{{.Name}}")
 
-	if r.Method != "POST" {
-		err := ErrorWithCause(ErrWebrpcBadMethod, fmt.Errorf("unsupported method %q (only POST is allowed)", r.Method))
-		RespondWithError(w, err)
-		return
-	}
-
+	var handler func(ctx context.Context, w http.ResponseWriter, r *http.Request)
 	switch r.URL.Path {
 	{{- range .Methods}}
-	case "/rpc/{{$name}}/{{.Name}}":
-		s.serve{{.Name | firstLetterToUpper}}(ctx, w, r)
-		return
+	case "/rpc/{{$name}}/{{.Name}}": handler = s.serve{{.Name | firstLetterToUpper}}JSON
 	{{- end}}
 	default:
 		err := ErrorWithCause(ErrWebrpcBadRoute, fmt.Errorf("no handler for path %q", r.URL.Path))
 		RespondWithError(w, err)
 		return
 	}
-}
-{{range .Methods }}
-func (s *{{$serviceName}}) serve{{.Name | firstLetterToUpper}}(ctx context.Context, w http.ResponseWriter, r *http.Request) {
-	header := r.Header.Get("Content-Type")
-	i := strings.Index(header, ";")
-	if i == -1 {
-		i = len(header)
+
+	if r.Method != "POST" {
+		w.Header().Add("Allow", "POST") // RFC 9110.
+		err := ErrorWithCause(ErrWebrpcBadMethod, fmt.Errorf("unsupported method %q (only POST is allowed)", r.Method))
+		RespondWithError(w, err)
+		return
 	}
 
-	switch strings.TrimSpace(strings.ToLower(header[:i])) {
+	contentType := r.Header.Get("Content-Type")
+	if i := strings.Index(contentType, ";"); i >= 0 {
+		contentType = contentType[:i]
+	}
+	contentType = strings.TrimSpace(strings.ToLower(contentType))
+
+	switch contentType  {
 	case "application/json":
-		s.serve{{ .Name | firstLetterToUpper }}JSON(ctx, w, r)
+		handler(ctx, w, r)
 	default:
 		err := ErrorWithCause(ErrWebrpcBadRequest, fmt.Errorf("unexpected Content-Type: %q", r.Header.Get("Content-Type")))
 		RespondWithError(w, err)
 	}
 }
-
+{{range .Methods }}
 func (s *{{$serviceName}}) serve{{ .Name | firstLetterToUpper }}JSON(ctx context.Context, w http.ResponseWriter, r *http.Request) {
 	var err error
 	ctx = context.WithValue(ctx, MethodNameCtxKey, "{{.Name}}")