def access_token
          if @notification.app.access_token.nil? || @notification.app.access_token_expired?
            post = Net::HTTP::Post.new(WPN_TOKEN_URI.path, 'Content-Type' => 'application/x-www-form-urlencoded')
            post.set_form_data(ACCESS_TOKEN_REQUEST_DATA.merge('client_id' => @notification.app.client_id, 'client_secret' => @notification.app.client_secret))