gistfile1.rb

class ExpiresXHRRequests
  def initialize(app)
    @app = app
  end
  
  def call(env)    
    is_xhr = env['HTTP_X_REQUESTED_WITH'] == 'XMLHttpRequest'  
    status, headers, body = @app.call(env)    
    if is_xhr && !headers.has_key?('Expires') # only set if NOT already set
      headers['Expires'] = "#{one_week_ago.strftime('%a, %d %b %Y %H:%M:%S GMT')}"
    end
    [status, headers, body]
  end
  
  private
  
  def one_week_ago
    Time.now - (60 * 60 * 24 * 7)
  end

end