Fix for issue #82

[afdev82]

I've spent some time to dig into the code. At least for me, these changes fixed the issue #82. Since I'm not familiar with the source code, I don't know if this is a good solution and if it would work for other DBMS (e.g. MySQL). I've tested it with my application using a dropdown filter and a Postgresql DB. Actually I'm using the branch compatible with Rails 6.1 from kreintjes repo. I haven't applied the same change to all processors, because I wanted to have a feedback before.