reverseproxy: Set cookie path to / when using cookie lb_policy

[d-masson]

When using cookie lb_policy, the cookie sent doesn't contain a path. So the path of the cookie stored by the browser is the url path

For exemple with this Caddyfile:

{
 debug
 admin localhost:2010
 auto_https disable_redirects
}
http://localhost:8443 {
    respond "B"
}
http://localhost:8444 {
    respond "C"
}
http://localhost:8442 {
    reverse_proxy /* {
        to http://localhost:8443 http://localhost:8444
        lb_policy cookie 
    }
}

1. When using url http://localhost:8442/A/B/C, caddy send a cookie without path and this cookie is stored in the browser with the path /A/B
2. When the same brower try to reach url http://localhost:8442/A/E/D, the browser doesn't send the first cookie (as the path doesn't match), so the server is sending a new cookie without path, the stickyness is no more guarantee and the browser store the same cookie a second time with a different path (/A/E)

=> We should always send the path / in the cookie

[francislavoie]

This is a followup to #3809

Thanks!

[mholt]

Thanks for the proactive fix!