Exercise 1: A Real Logout via delete_cookie() — Possible Solution
====================================================================

THE METHOD AND THE ROUTE
------------------------------
    def delete_cookie(self, name, path='/'):
        cookie = SimpleCookie()
        cookie[name] = ''
        cookie[name]['path'] = path
        cookie[name]['max-age'] = 0
        self.headers.append(('Set-Cookie', cookie[name].OutputString()))

    def logout_handler(request):
        request.state['logged_out'] = True
        response = Response("logged out")
        response.delete_cookie('session_id')
        return response

The session middleware itself needs one small change so it doesn't
immediately re-set the cookie it was just told to delete:

    def middleware(request, next_):
        ...
        response = next_(request)
        store.save(sid, request.state['session'])
        if not request.state.get('logged_out'):
            response.set_cookie('session_id', sid, max_age=cookie_max_age)
        return response

VERIFIED, REAL RESULT
------------------------------
    request 1:              b'You have visited 1 times'
    request 2:               b'You have visited 2 times'
    logging out:              b'logged out'
    cookies jar holds after logout: []
    request after logout:     b'You have visited 1 times'

A real Max-Age=0 Set-Cookie header genuinely causes http.cookiejar to
remove the cookie from its own jar entirely - confirmed directly by
listing the jar's contents right after the logout request and finding
it empty. The very next request has no session_id to send at all, so
the middleware creates a real, brand-new session from scratch.

WHY THIS WORKS AS AN ANSWER
------------------------------
It reuses set_cookie()'s own SimpleCookie machinery for the opposite
job - clearing a cookie is just a Set-Cookie with an empty value and
Max-Age=0, no separate mechanism needed - and verifies the real,
client-side consequence (the jar actually drops the cookie) rather
than only checking the header text was sent correctly.