Refactor whereRaw to use whereColumn (#25)

Addresses the issue with json field relationship. E.g
{
"object_type": "comment",
"object_id": "1"
} 

$this->morphOne(Notification::class, 'data->object';)

src/Morph.php

@@ -115,9 +115,7 @@ protected function queryCleanOrphan(Model $parentModel, Relation $relation, bool
                 ) {
                     $query->select(DB::raw(1))
                             ->from($parentModel->getTable())
-                            ->whereRaw(
-                                $parentModel->getTable().'.'.$parentModel->getKeyName().' = '.$childTable.'.'.$childFieldId
-                            );
+                            ->whereColumn($parentModel->getTable() . '.' . $parentModel->getKeyName(), '=', $childTable . '.' . $childFieldId);
                 })->$method();
     }