Some tests are failing on MySQL8 and MariaDB

[Weakky]

Overview

Since we added MySQL8 and MariaDB to the tested databases (https://github.com/prisma/quaint/pull/315), a couple of tests are now failing. Namely:

• [ ] newdecimal_conversion_is_handled_correctly
• [ ] filtering_by_json_values_does_not_work_but_does_not_crash
• [ ] text_columns_with_non_utf8_encodings_can_be_queried
• [x] json_array_contains_fun (fixed by #319)
• [x] json_array_begins_with_fun (fixed by #319)
• [x] json_array_not_begins_with_fun (fixed by #319)
• [x] json_array_ends_into_fun (fixed by #319)
• [x] json_array_not_ends_into_fun (fixed by #319)

These all need to be fixed.