Collections - support for sets and symbols

I'm using the role_model gem to model roles in my application. RoleModel exposes a 'roles' attribute which is implemented as a set of symbols. I tried creating a checkboxes input, but Formtastic just ignored the selected values.

I modified Formtastic input collections to accept sets (or any array-like object that responds to :to_a). I also modified it to work with symbols.