fix for grpc client

mount.go

@@ -85,7 +85,15 @@ func (m *Mounter) Handler(f interface{}) http.Handler {
 	numIn := ft.NumIn()
 	mapIn := make(map[mapIndex]int)
 	for i := 0; i < numIn; i++ {
-		switch ft.In(i).String() {
+		fi := ft.In(i)
+
+		// assume this is grpc call options
+		if fi.Kind() == reflect.Slice && i == numIn-1 {
+			numIn--
+			break
+		}
+
+		switch fi.String() {
 		case strContext:
 			setOrPanic(mapIn, miContext, i)
 		case strRequest:

mount_test.go

@@ -133,6 +133,15 @@ func TestHandler(t *testing.T) {
 	reset()
 	h.ServeHTTP(w, r)
 	mustNothing()
+
+	// grpc style
+	h = m.Handler(func(ctx context.Context, req *requestType, opts ...interface{}) (interface{}, error) {
+		return map[string]string{"ok": "1"}, nil
+	})
+	r = httptest.NewRequest(http.MethodPost, "http://localhost", successBody)
+	reset()
+	h.ServeHTTP(w, r)
+	mustSuccess()
 }
 
 func TestDefault(t *testing.T) {