Cast enums in lt(), lte(), gt(), gte()

[Expurple]

Revert 10f3de0, this was discussed under #1527. There weren't any tests for these operators in that initial PR (#258). Let me know if mine can't be merged without adding some. If that's the case, I'm not sure where they should go, bloating tests::active_enum_tests::insert_active_enum doesn't feel right.

[tyt2y3]

Thank you. Yes, tests are important. I guess the best way is probably for you to add a new table under tests/common/features.

[Expurple]

Ok, so I tried to add a new table and use it for tests. Then I realized that comparison results depend on the enum's db_type annotation (whether it's an Integer, String or Enum) and that existing ActiveEnum table conveniently contains columns for all three enum representations. So I ended up using that.

Postgres and SQLite look fine, but I got surprising results for MySQL (see the code comments). If you don't mind, I would like to merge this as is to get Postgres working and create a separate issue for MySQL, which I don't want to dig into at the moment.

[tyt2y3]

Happy new year and thank you!