This commit fixes the following PHPStan error what started happening
after the update to version 0.12.83. It seems that our model classes
work with properties using both snake_case and camelCase names, so that
is why the change made here doesn't affect the modified unit test.
```
------ ---------------------------------------------
Line tests/integration/Models/SubscriberTest.php
------ ---------------------------------------------
740 Access to an undefined property
MailPoet\Models\Subscriber::$deleted_at.
------ ---------------------------------------------
```
[MAILPOET-3491]