Rake task to export demo site data doubles all tag counts

[denny]

To reproduce:

1. Run rails shinycms:demo:load to reset the database and load the demo data, including some tag data
2. Run rails shinycms:demo:export to export the database content to db/demo_site_data.rb
3. Run git diff (also, compare the tag.taggings data in the database to the data in db/demo_site_data.rb)