[5.4] Default foreign key for belongsTo relationship is now dynamic

[ElMatella]

Following #16396 in order to make foreign keys more dynamics. Before, you needed to reference foreign key on belongsTo relationship if primary key of relation was not id:

class Foo extends Model 
{
    protected $primaryKey = 'key';
}

class Bar extends Model
{
    public function foo () 
    {
        return $this->belongsTo(Foo::class,'foo_key');
    }
}

Now, you should be able to just write:

class Bar extends Model
{
    public function foo () 
    {
        return $this->belongsTo(Foo::class);
    }
}