Use default-path in Plush cookies.

Cookies are not port specific. When sending cookies with path=/ to
http://localhost:8090/sabnzbd/ these cookies also turn up when
requesting e.g. http://localhost/apache2 or http://localhost:8080/tomcat and so on.

To avoid polluting the global cookie space, simply do not specify a path
when setting the cookie. This will result in a cookie with the default-path,
i.e. path=/sabnzbd for http://localhost:8090/sabnzbd.