fix: Support embedded struct

[mtt0]

With this fix, the following code will output the expected result, tested on MaxOS and Linux:

go get -u github.com/mtt0/dynamic-struct@0e75835bc641757dac062453629f1300ea57678c

• go.mod

module main

go 1.13

require github.com/mtt0/dynamic-struct v1.3.1-0.20220614084457-0e75835bc641 // indirect

• main.go

package main

import (
	"encoding/json"
	"fmt"
	"log"

	//dynamicstruct "github.com/Ompluscator/dynamic-struct"
	dynamicstruct "github.com/mtt0/dynamic-struct"
)

type A struct {
	B
	String string `json:"str"`
}

type B struct {
	Name string `json:"name"`
}

func main() {
	instance := dynamicstruct.ExtendStruct(A{}).
		Build().
		New()
	data := []byte(`
{
    "str": "string in a",
    "name": "string in b"
}
`)
	err := json.Unmarshal(data, &instance)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(instance)
	// Out:
	// &{{} string in a}
	//
	// Expected:
	// &{{string in b} string in a}

	data, err = json.Marshal(instance)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(string(data))
	// Out:
	// {"B":{"name":""},"str":"string in a"}
	//
	// Expected:
	// {"name":"string in b","str":"string in a"}
}