[5.3] Prevent calling Model.php methods when calling them as attributes

[themsaid]

When $user->delete is called, Eloquent will try to get an attribute with name delete, if not found it'll assume it's a name of a relation and try to call the method delete which will throw an exception Relationship method must return an object of type ... however the delete() method will be executed.

This PR checks if the attribute has the name of a method existing in Model.php it returns null and doesn't assume it's a relationship.

[taylorotwell]

How would you access any relationships if this change is implemented?

[themsaid]

@taylorotwell method_exists(self::class, $key) will check for the existence of the method in the parent Model class only.

I've added more tests to make sure the case for calling a relation is covered.

[taylorotwell]

Would it still work if you were defining a relation on a model you are extending?

[themsaid]

Yes, having method_exists(self::class, $key) in Model.php means that the check will only look for methods in that specific abstract class Illuminate\Database\Eloquent\Model.

[taylorotwell]

I guess I'm confused again. If it only looks in the abstract class (Model), how would it find relations defined on my own model (App\User)?