Support primary keys that aren't named id #154
Description
I understand that when giving a order_by: column other than the default :id, extra care is required to ensure that unique results are accounted for because we cannot ensure that the user specified column is unique, and hence adding :id is required. However, I am thinking that the 'spirit' of this behavior is really not about :id specifically but about if we are sorting by the table's primary key or not. I have a few cases where unfortunately the table's primary key is not called :id, and in fact also does not even have a column named :id, and hence the 'more complex' sorting case is triggered and fails because there is no :id column.
I may or may not take a swing at this change if it seems worthwhile, but at a glance it seems that in Paginator something like order_by ||= :id could be replaced with order_by ||= relation.primary_key.to_sym, and then a few more spots also would have to be updated where we are relying on the behavior that the primary key is :id.