Rails 6.1 compatibility

[SofiaSousa]

Adds an override to ActiveRecord update method to set the dynamic attributes, since update_attributes methods are deprecated in Rails 6.1.

[moiristo]

Hi! Thanks for your PR! Unfortunately, I haven't maintained this gem in 11 years though and I'm not using it anymore.
That being said, it's fine with me to merge this, although I don't think I will be able to release a new version because of this. Please let me know if this is ok. If you like I can also make you a collaborator, so you can push changes yourself.

[SofiaSousa]

Thanks for your fast reply!

We have been using this gem for several years 👏 . Now we are upgrading our application and we need this fix to keep it working as it was.

Actually, we've already discussed/planned to replace the gem in the future after the upgrade, so it's not a big problem to use the fork we've created in meantime 🙂 👍 .