[Eloquent] Don't use hard-coded primary key column

In two small cases, the ID was still hard-coded in the Fluent class.

This was leading to problems, as Eloquent makes use of the Fluent class, but wasn't "overwriting" these methods.

@taylorotwell While we're at it: could you explain why you chose this weird construct using `__call()` to pass methods to the normal database query (Fluent) class instead of simply extending the Fluent query class for Eloquent queries?