fix custom class cast with dates

Author: taylorotwell

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

@@ -231,6 +231,11 @@ protected function addCastAttributesToArray(array $attributes, array $mutatedAtt
                 $attributes[$key] = $attributes[$key]->format(explode(':', $value, 2)[1]);
             }
 
+            if ($attributes[$key] && $attributes[$key] instanceof DateTimeInterface &&
+                $this->isClassCastable($key)) {
+                $attributes[$key] = $this->serializeDate($attributes[$key]);
+            }
+
             if ($attributes[$key] instanceof Arrayable) {
                 $attributes[$key] = $attributes[$key]->toArray();
             }