[5.4] Use fromRawAttributes when creating a newPivot

[PhiloNL]

Casting attributes stored as JSON don't work properly when using a pivot model and setting the $casts property.

In 5.2 fromRawAttributes method was added to manually resolve this issue. #13294

When using the new "using" method, for example: $this->belongsToMany(Team::class, 'user_teams', 'user_id', 'team_id')->using(UserTeamPivot::class), the JSON is still encoded twice. It would be great if this could be changed so the fromRawAttributes method is used.

class UserTeamPivot extends Pivot
{
    protected $casts = ['permissions' => 'collection'];
}

// App\User::find(1)->teams->first()
 App\Team {#1437
     id: 1,
     name: "Team Name",
     pivot: App\UserTeamPivot {#1436
       user_id: 1,
       team_id: 1,
       permissions: ""[\"create_campaign\"]"",
     },
   }

When using the fromRawAttributes method this is resolved.

// App\User::find(1)->teams->first()
 App\Team {#1437
     id: 1,
     name: "Team Name",
     pivot: App\UserTeamPivot {#1436
       user_id: 1,
       team_id: 1,
       permissions: "["create_campaign"]",
     },
   }

[ntzm]

I guess this also needs to be changed here?