nested field with json tag 'omitempty' should not be marshalled if it has zero value

[gfremex]

Example code like this:

package main

import (
	"encoding/json"
	"fmt"

	"github.com/json-iterator/go"
)

func main() {
	type S1 struct {
		F1 string `json:",omitempty"`
	}

	type S2 struct {
		*S1
		F2 string `json:",omitempty"`
	}

	s1 := &S1{
		//F1: "abc",
	}

	s2 := &S2{
		S1: s1,
		F2: "123",
	}

	b, err := json.Marshal(s2)

	if err != nil {
		panic(err)
	}

	fmt.Println("Marshalled by json:", string(b))

	b, err = jsoniter.Marshal(s2)

	if err != nil {
		panic(err)
	}

	fmt.Println("Marshalled by jsoniter:", string(b))
}

And the output is:

Marshalled by json: {"F2":"123"}
Marshalled by jsoniter: {"F1":"","F2":"123"}