def upgrade!
      access_token = nil
      transaction do
        access_token = OauthProviderEngine::AccessToken.create!({
          :application_id => self.application_id,
          :user_id => self.user_id,
        })
        self.destroy || raise(ActiveRecord::Rollback)
      end
      return access_token
    end
def upgrade!
      access_token = nil
      transaction do
        access_token = OauthProviderEngine::AccessToken.create!({
          :application_id => self.application_id,
          :user_id => self.user_id,
        })
        self.destroy || raise(ActiveRecord::Rollback)
      end
      return access_token
    end
this method with upgrade the RequestToken to an AccessToken
   note that this will destroy the current RequestToken