replacing tags doesn't persist in the database.

replace-tags.rb

# loop through article containing the old_tag
Article.tagged_with(old_tag).each do |article|
  # give articles with the old tag the new tag
  article.tag_list.add(new_tag)
  # remove the old tag
  article.tag_list.remove(old_tag)

  article.save # this isn't 
end

# is this a similar problem? https://github.com/mbleigh/acts-as-taggable-on/issues/229