Add unit tests for relation methods

- [x] except
- [x] destroy_all
- [x] empty?
- [x] exists?
- [x] find
- [x] limit
- [x] offset
- [x] order
- [x] pluck
- [x] reorder
- [x] with_context
- [x] where
- [x] uniq

Dynamic finders
- [x] find_all_by
- [x] find_by