Collapse multiple X-Forwarded-For headers

Fixes #1650

bin/varnishd/cache/cache_req_fsm.c

@@ -724,6 +724,7 @@ cnt_recv(struct worker *wrk, struct req *req)
 		 * This really should be done earlier, but we want to capture
 		 * it in the VSL log.
 		 */
+		http_CollectHdr(req->http, H_X_Forwarded_For);
 		if (http_GetHdr(req->http, H_X_Forwarded_For, &xff)) {
 			http_Unset(req->http, H_X_Forwarded_For);
 			http_PrintfHeader(req->http, "X-Forwarded-For: %s, %s",

bin/varnishtest/tests/r01650.vtc

+varnishtest "xff handling discards multiple headers"
+
+server s1 {
+	rxreq
+	expect req.http.X-Forwarded-For == "1.2.3.4, 5.6.7.8, 127.0.0.1"
+	txresp
+} -start
+
+varnish v1 -vcl+backend {
+} -start
+
+client c1 {
+	txreq -hdr "X-Forwarded-For: 1.2.3.4" -hdr "X-Forwarded-For: 5.6.7.8"
+	rxresp
+} -run
+