Allow unmarshalling slices.Map to nil map

[ThePrzeglo]

I've tried to bind from JSON request struct:

cmt := Comment{
    // Data: slices.Map{}, // after uncomment works
}
c.Bind(&cmt)

type Comment struct {
	ID        uuid.UUID  `json:"id" db:"id"`
	CreatedAt time.Time  `json:"created_at" db:"created_at"`
	UpdatedAt time.Time  `json:"updated_at" db:"updated_at"`
	Data      slices.Map `json:"data" db:"data"`
}

And it turned out that it crashes on slices.Map on line 51 with assignment to entry in nil map
https://github.com/gobuffalo/pop/blob/master/slices/map.go#L51
Small change to UnmarshalJSON seems to fix issue:

func (m *Map) UnmarshalJSON(b []byte) error { // ptr here
	var stuff map[string]interface{}
	err := json.Unmarshal(b, &stuff)
	if err != nil {
		return err
	}
	if *m == nil { // if map that this pointer points to is nil:
		*m = Map{} //  allocate new one
	}
	for key, value := range stuff {
		(*m)[key] = value
	}
	return nil
}