Unable to add hydra_attributes to hydra_sets

[matt-morris]

I'm running into the following error trying to use hydra_sets:

NoMethodError: undefined method `hydra_attributes=' for #<HydraAttribute::HydraSet:0x007fe757f5be28>
from /Users/me/hydra_project/.bundle/ruby/2.1.0/bundler/gems/hydra_attribute-b17875a46560/lib/hydra_attribute/model/persistence.rb:411:in `method_missing'

Here's the code leading up to the issue:

birthdate = User.hydra_attributes.create(name: 'birthdate', backend_type: 'datetime')
birthday_club = User.hydra_sets.create(name: 'birthday_club')
birthday_club.hydra_attributes = [birthdate]

The project is using Rails 4.1.1 on ruby 2.1.0.

cc: @chrishough

[leandroo]

Same error here, I'm using Rails 3.2.16 on Ruby 1.9.3