Redirecting open-method for rspec test that I dont have to rely on availability of an API-Server

user.rb

require 'nokogiri'
require 'open-uri'

def User < ActiveRecord::Base
  def update_user
    xml = Nokogiri::XML(open("http://example.com/api/#{pkey}/#{ukey}"))

    self.name    = xml.xpath('hordes/headers/owner/citizen').first['name']
    self.avatar  = xml.xpath('hordes/headers/owner/citizen').first['avatar']
    self.last_update = Time.now
    self.save
    self
  end
end