Fix performSearch.

Use actual model's key name instead of hardcoded 'id' value in the order by clause.

Author: pmatseykanets

src/PostgresEngine.php

@@ -216,7 +216,7 @@ protected function performSearch(Builder $builder, $perPage = 0, $page = 1)
             ->selectRaw('COUNT(*) OVER () AS total_count')
             ->whereRaw("$indexColumn @@ query")
             ->orderBy('rank', 'desc')
-            ->orderBy('id');
+            ->orderBy($builder->model->getKeyName());
 
         if ($perPage > 0) {
             $query->skip(($page - 1) * $perPage)