Cannot access camelCase relationship as a property (again)

My other issue was closed, but the bug remains, so I'm opening another one to track it. Please refer to the description in the other issue.

https://github.com/laravel/framework/issues/8241

As a summary, these are the exact same internal definition, just with a different relationship name:

``` php
$object->camelCase->toArray(); // fatals
$object->snake_case->toArray(); // works
```
