A valid Gost-Target / X-Gost-Target header replaced only req.Host while
leaving req.URL.Host untouched. Destination-policy checks (bypass) evaluated
the header-derived authority, but http.Transport dialed the unchanged
absolute-URL authority. A client could label a forbidden URL with an allowed
authority and reach the blocked destination (403 -> 200 with backend body).
setRequestHost now applies the decoded target to both req.Host and req.URL.Host
so policy evaluation and transport dialing always resolve to the same authority.