Must be empty Rule doesn't work on Hidden Fields

I agree with that, I use an empty hidden input "mail" for a while and it works for most of spamming cases. It filtered spammed contact forms better than I thought.
So the patch below allows to use hidden fields for "must be empty" validation. Il is more simple than use a textfield and hide it within CSS rules.

The "Must be empty" validation rule can now be applied to hidden components.
Although I think spambots still mainly want to fill in normal fields, and will skip hidden fields, as there's no user input possible in the first place...
Anyway, it's there now, so use it as you wish :)

Thanks!
I confirm that this trick works like a charm for most of sites. Just do not use it alone for big Websites, because in such a case the spammers are really more motivated ;-)