workaround for not creating tagging with no tags

- changing the tags relationship on before validation messes up with
  the taggings one
- for each tag that was already created, two tagging records were being
  created, one correct and one with no tag_id:

  #<ActiveRecord::Associations::CollectionProxy [
    #<Tag id: 973456317, name: "tag#1", ... >
  ]>
  #<ActiveRecord::Associations::CollectionProxy [
    #<Tagging id: nil, tag_id: nil, link_id: nil, ... >,
    #<Tagging id: nil, tag_id: 973456317, link_id: nil, ...
  ]>

- after cleaning up taggings relationship before changing the tags we
  got the correct result:

  #<ActiveRecord::Associations::CollectionProxy [
    #<Tag id: 973456319, name: "tag#1", ... >
  ]>
  #<ActiveRecord::Associations::CollectionProxy [
    #<Tagging id: nil, tag_id: 973456319, link_id: nil, ... >
  ]>