Make Memory::Query#offset behave like SQL

Right now, the `offset(number)` method in `Adapter::Memory::Query` returns the last `number` records. The normal behaviour for SQL is to skip the first `number` records. E.g. `offset(1)` with the dataset `[user1, user2, user3]` should result in a query that returns `[user2, user3]`, not `[user3]`.

This fix makes the `offset` in the memory adapter behave more like the method in the SQL adapter.