Do not send a zero Content-Length for 204 and 304

In the 304 case we were already skipping it for non-zero C-L.
For 204 responses this was introduced as a side effect of 271e1c52.

bin/varnishd/cache/cache_req_fsm.c

@@ -73,7 +73,7 @@ cnt_vdp(struct req *req, struct busyobj *bo)
 		else
 			sendbody = 0;
 	} else if (status < 200 || status == 204 || status == 304) {
-		req->resp_len = 0;
+		req->resp_len = -1;
 		sendbody = 0;
 	} else
 		sendbody = 1;

bin/varnishtest/tests/c00008.vtc

@@ -6,6 +6,10 @@ server s1 {
 	txresp -hdr "Last-Modified: Thu, 26 Jun 2008 12:00:01 GMT" \
 	    -hdr {ETag: "foo"} \
 	    -body "11111\n"
+	rxreq
+	expect req.url == "/bar"
+	txresp -hdr "Last-Modified: Thu, 26 Jun 2008 12:00:01 GMT" \
+	    -hdr {ETag: "bar"}
 } -start
 
 varnish v1 -vcl+backend { } -start
@@ -41,6 +45,21 @@ client c1 {
 	expect resp.http.etag == {"foo"}
 	expect resp.http.content-length == "<undef>"
 	expect resp.bodylen == "<undef>"
+
+	txreq -url "/bar"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.etag == {"bar"}
+	expect resp.http.content-length == "0"
+	expect resp.bodylen == 0
+
+	txreq -url "/bar" \
+	    -hdr "If-Modified-Since: Thu, 26 Jun 2008 12:00:01 GMT"
+	rxresp -no_obj
+	expect resp.status == 304
+	expect resp.http.etag == {"bar"}
+	expect resp.http.content-length == <undef>
+	expect resp.bodylen == <undef>
 }
 
 client c1 -run

bin/varnishtest/tests/r01826.vtc

 varnishtest "Check we ignore a zero C-L with a 204"
 
 server s1 {
+	rxreq
+	txresp -status 204 -bodylen 5
+	expect_close
+
+	accept
 	rxreq
 	txresp -status 204
 } -start
@@ -9,7 +14,12 @@ varnish v1 -vcl+backend {
 } -start
 
 client c1 {
+	txreq
+	rxresp
+	expect resp.status == 503
+
 	txreq
 	rxresp
 	expect resp.status == 204
+	expect resp.http.content-length == <undef>
 } -run