change array_merge to array_replace to prevent reindex (#45309)

Fixes issue where array [506 => null, 'name' => 'kevin'] is reindexed as [0 => null, 'name' => 'kevin']

src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php

@@ -1119,7 +1119,7 @@ protected function setClassCastableAttribute($key, $value)
     {
         $caster = $this->resolveCasterClass($key);
 
-        $this->attributes = array_merge(
+        $this->attributes = array_replace(
             $this->attributes,
             $this->normalizeCastClassResponse($key, $caster->set(
                 $this, $key, $value, $this->attributes