Add back in attr_accessible to delayed ActiveRecord object

Otherwise delayed_job ActiveRecord::AttributeAssginment::ActiveSupport::concern fails in Rails 3 with an error such as:

Can't mass-assign protected attributes: priority, queue, payload_object

I traced this back to https://github.com/collectiveidea/delayed_job_active_record/commit/0cb4496ce75f7edad2807dc4b118949a335e421e so re-added attr_accessible and the accompanying tests as it seems safe for the context in which the class is used.

I do not have rails 4 installed anywhere, so have not confirmed yet that this is rails 4 compatible.