def call(env)
      session = Clearance::Session.new(env)
      env[:clearance] = session
      response = @app.call(env)
      session.add_cookie_to_headers response[1]
      response
    end
def call(env)
      session = Clearance::Session.new(env)
      env[:clearance] = session
      response = @app.call(env)
      session.add_cookie_to_headers response[1]
      response
    end
Reads previously existing sessions from a cookie and maintains the cookie
 on each response.