Your user.action excerpt lacks an URL pattern.

If you check your test URL with, you'll probably see that the filter isn't applied.

You could also enable the debug directives suggested at and check the log.

Adding an URL pattern should enable the filter.

There currently isn't a "proper" way to add a server header, so rewriting an existing header is the usual method. You can also append a new header to an existing header by using "\r\n". This is an undocumented "feature", though, and Privoxy itself will treat the two headers as one (changing this is TODO item #112).