Fix self referential BelongsTo (again)

The test was passing only because UserFactory didn't exist.

src/ModelPopulator.php

@@ -198,7 +198,7 @@ protected function associateBelongsTo(array &$formatters, BelongsTo $relation):
         }
 
         // Skip foreign keys for relations of the model to itself to prevent infinite recursion.
-        if ($relatedClass instanceof $this->model) {
+        if ($this->model instanceof $relatedClass) {
             $formatters[$foreignKey] = null;
 
             return;

tests/Factories/UserFactory.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace GuidoCella\EloquentPopulator\Factories;
+
+use GuidoCella\EloquentPopulator\Models\User;
+use Illuminate\Database\Eloquent\Factories\Factory;
+
+class UserFactory extends Factory
+{
+    protected $model = User::class;
+
+    public function definition()
+    {
+    }
+}