Fix data extract failure

Upgrade to rails 3.2.11 broke the data extract task; the
composite_primary_key gem dependency wasn't suitable and the task
failed

/data/vhost/efg.production.alphagov.co.uk/shared/bundle/ruby/1.9.1/gems/activemodel-3.2.11/lib/active_model/attribute_methods.rb:407:in `method_missing'
        /data/vhost/efg.production.alphagov.co.uk/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.11/lib/active_record/attribute_methods.rb:149:in `method_missing'
        /data/vhost/efg.production.alphagov.co.uk/shared/bundle/ruby/1.9.1/gems/composite_primary_keys-5.0.9/lib/composite_primary_keys/attribute_methods/dirty.rb:21:in `write_attribute'
        /data/vhost/efg.production.alphagov.co.uk/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.11/lib/active_record/attribute_methods/write.rb:14:in `user_id='
        /data/vhost/efg.production.alphagov.co.uk/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.11/lib/active_record/attribute_assignment.rb:85:in `block in assign_attributes'
        /data/vhost/efg.production.alphagov.co.uk/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.11/lib/active_record/attribute_assignment.rb:78:in `each'
        /data/vhost/efg.production.alphagov.co.uk/shared/bundle/ruby/1.9.1/gems/activerecord-3.2.11/lib/active_record/attribute_assignment.rb:78:in `assign_attributes'
        /data/vhost/efg.production.alphagov.co.uk/shared/bundle/ruby/1.9.1/bundler/gems/data-anonymization-646ffa6be9ac/lib/strategy/whitelist.rb:19:in `process_record'

Copied a test from the underlying data-anonymization gem and also
reproduced the mass assignment issue in the process; we can now
provide a single spec test case to the upstream issue.