Shouldn't the slices.Map receiver for UnmarshalJSON be a pointer?

[naemono]

Description

It seems as though unmarshaling to a struct that contains a slices.Map doesn't work as expected, and the only solution is to adjust the receiver for Map.UnmarshalJSON to be a pointer, as below shows

package main

import (
	"encoding/json"
	"log"

	"github.com/gobuffalo/pop/v5/slices"
)

type Account struct {
	Credentials slices.Map `json:"credentials" db:"credentials"`
}

func main() {
	m := map[string]interface{}{
		"credentials": map[string]interface{}{
			"test": "data",
		},
	}
	b, err := json.Marshal(&m)
	account := &Account{}
	json.Unmarshal(b, account)
	log.Printf("err: %v, account: %v", err, account)
}

you'll see this

$ go run map.go
2021/06/11 15:24:13 err: <nil>, account: &{map[]}

If I update UnmarshalJSON to be

func (m *Map) UnmarshalJSON(b []byte) error {
	var stuff map[string]interface{}
	err := json.Unmarshal(b, &stuff)
	if err != nil {
		return err
	}
	if *m == nil {
		*m = Map{}
	}
	for key, value := range stuff {
		(*m)[key] = value
	}
	return nil
}

like here https://github.com/naemono/pop/tree/map-unmarshal-pointer-receiver

$ go mod edit -replace github.com/gobuffalo/pop/v5=github.com/naemono/pop/v5@map-unmarshal-pointer-receiver                                                                                                                                                        
$ go run map.go
go: downloading github.com/naemono/pop/v5 v5.3.2-0.20210611203100-4a801ab183ef
2021/06/11 15:38:05 err: <nil>, account: &{map[test:data]}

I'll open a PR shortly referencing this issue

Steps to Reproduce the Problem

See above

Expected Behavior

I'd expect the map data to be contained within the slices.Map object with a struct during json.Unmarshal

Actual Behavior

map is empty

Info

If it's relevant

$ go version
go version go1.15 darwin/amd64