def post_update
      raise StateError.new 'device access_token prohibited' if @session.device?
      post_data = @data.dup
      post_data['tags'] = post_data.delete 'name'
      grok_self_from post 'tag/permissions/update', post_data
      self
    end
def post_update
      raise StateError.new 'device access_token prohibited' if @session.device?
      post_data = @data.dup
      post_data['tags'] = post_data.delete 'name'
      grok_self_from post 'tag/permissions/update', post_data
      self
    end
POST the tag's +@data+ to the API via 'tag/permissions/update', and return
 the same object with the new +@data+ returned from API call.