Fix encode issue for embed struct with tags

[peterlimg]

Embed struct with tag does not have the key encoded, for example:

type Foo struct {
    Name string
}
type Bar struct {
	Foo `json:"foo"`
	ID  string `json:"id"`
}
func main() {
	b := Bar{ID:"abc"}
	v, err := json.Marshal(b)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(string(v))
}

The output will be {{"Name":""},"id":"abc"}, the key of foo is lost, which is different from the behavior of encoding/json.

This PR will fix this issue.

[codecov-commenter]

Codecov Report

Merging #265 (91e691a) into master (85a5597) will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
+ Coverage   81.00%   81.24%   +0.23%     
==========================================
  Files          47       47              
  Lines       14828    15566     +738     
==========================================
+ Hits        12012    12646     +634     
- Misses       2262     2360      +98     
- Partials      554      560       +6     

[goccy]

Thank you for the great PR !!! I appreciate it very much