Permit subsequent conditional requests on 304

Fixes #2871.

bin/varnishd/cache/cache_fetch.c

@@ -714,7 +714,8 @@ vbf_stp_condfetch(struct worker *wrk, struct busyobj *bo)
 		    OA_ESIDATA));
 
 	AZ(ObjCopyAttr(bo->wrk, bo->fetch_objcore, bo->stale_oc, OA_FLAGS));
-	ObjSetFlag(bo->wrk, bo->fetch_objcore, OF_IMSCAND, 0);
+	if (bo->fetch_objcore->flags & OC_F_HFM)
+		ObjSetFlag(bo->wrk, bo->fetch_objcore, OF_IMSCAND, 0);
 	AZ(ObjCopyAttr(bo->wrk, bo->fetch_objcore, bo->stale_oc, OA_GZIPBITS));
 
 	if (bo->do_stream) {

bin/varnishtest/tests/b00039.vtc

@@ -6,6 +6,9 @@ server s1 {
 	rxreq
 	expect req.http.if-modified-since == "Wed, 11 Sep 2013 13:36:55 GMT"
 	txresp -status 304
+	rxreq
+	expect req.http.if-modified-since == "Wed, 11 Sep 2013 13:36:55 GMT"
+	txresp -status 304
 } -start
 
 varnish v1 -vcl+backend {
@@ -44,3 +47,13 @@ client c1 {
 	expect resp.body == "Geoff Rules"
 	expect resp.http.was-304 == "true"
 } -run
+
+delay 1
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+	expect resp.body == "Geoff Rules"
+	expect resp.http.was-304 == "true"
+} -run