nested factories need their class to be defined, otherwise they use CustomField as a class
tests that issues won't have custom field errors duplicated
:validate is true by default. since we validate custom values a couple of lines below with #validates_associated (which also pulls the nested errors into the parent model for display above forms) we disable it in the #has_many call and remove the :on => :update in #validates_associated
fix failing tests: always trigger validation of custom fields, if any.