rproxy: remove forwarded headers spelt with underscores from an untrusted peer (closes #334)

For a peer outside `SWRP_TRUSTED_PROXIES`, `rproxy` now reads each header name with `_` as `-` and in any letter case, and removes the header when that name starts with `X-Forwarded-` or is `Forwarded` or `X-Real-IP`. Before, `X_Forwarded_Ssl: on` or `X_Real_IP` from a client connecting directly reached the app unchanged, and an app behind CGI or FastCGI reads them as the dash-spelt headers. A trusted peer's headers pass on as before. `SPEC.md` says so.

Model: opus-5-5

internal/rproxy/client.go

@@ -105,17 +105,16 @@ func setForwardedHeaders(in, out *http.Request, peer netip.Addr, trusted bool) {
 		return
 	}
 
-	// ReverseProxy has removed Forwarded and the three set below. Go's server
-	// gives header names in canonical form, so the prefix matches the name in
-	// any letter case.
+	// An app behind CGI or FastCGI reads X_Forwarded_Ssl as X-Forwarded-Ssl, so
+	// a name is matched with each _ read as - and in any letter case.
 	for name := range out.Header {
-		if strings.HasPrefix(name, "X-Forwarded-") {
+		spelt := strings.ToLower(strings.ReplaceAll(name, "_", "-"))
+		if strings.HasPrefix(spelt, "x-forwarded-") ||
+			spelt == "forwarded" || spelt == "x-real-ip" {
 			delete(out.Header, name)
 		}
 	}
 
-	out.Header.Del("X-Real-IP")
-
 	out.Header.Set("X-Forwarded-For", forwardedFor)
 	out.Header.Set("X-Forwarded-Host", in.Host)
 	out.Header.Set("X-Forwarded-Proto", "http")