refactor: fix lint errors; improve error handling

internal/route/rules/help.go

@@ -125,7 +125,7 @@ func helpVar(varExpr string) string {
 }
 
 /*
-Generate help string as error, e.g.
+Error generates help string as error, e.g.
 
 	rewrite <from> <to>
 		from: the path to rewrite, must start with /

internal/route/rules/parser.go

@@ -158,13 +158,14 @@ func parse(v string) (subject string, args []string, err error) {
 		buf.WriteRune('$')
 	}
 
-	if quote != 0 {
+	switch {
+	case quote != 0:
 		err = ErrUnterminatedQuotes
-	} else if brackets != 0 {
+	case brackets != 0:
 		err = ErrUnterminatedBrackets
-	} else if inEnvVar {
+	case inEnvVar:
 		err = ErrUnterminatedEnvVar
-	} else {
+	default:
 		flush(false)
 	}
 	if len(missingEnvVars) > 0 {

internal/route/rules/rules.go

@@ -286,8 +286,7 @@ func logError(err error, r *http.Request) {
 	var h2Err http2.StreamError
 	if errors.As(err, &h2Err) {
 		// ignore these errors
-		switch h2Err.Code {
-		case http2.ErrCodeStreamClosed:
+		if h2Err.Code == http2.ErrCodeStreamClosed {
 			return
 		}
 	}

internal/route/rules/vars.go

@@ -39,7 +39,7 @@ func NeedExpandVars(s string) bool {
 var (
 	voidResponseModifier = httputils.NewResponseModifier(httptest.NewRecorder())
 	dummyRequest         = http.Request{
-		Method: "GET",
+		Method: http.MethodGet,
 		URL:    &url.URL{Path: "/"},
 		Header: http.Header{},
 	}