header before adding our own.

We can not do this with the fetch filtering, because pass might send
HEAD requests to the backend.

XXX: It's arguable that the filtering stuff might be better done
inline than with a table.

Fixes: #801



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5461 d4fa192b-c00b-0410-8231-f00ffab90ce4