h2: Relax "no //" URLs requirement

This requirement was dropped in the updated rfc 9113.

Fixes: #3911

bin/varnishd/http2/cache_http2_hpack.c

@@ -134,10 +134,9 @@ h2h_addhdr(struct h2h_decode *d, struct http *hp, char *b, size_t namelen,
 			n = HTTP_HDR_URL;
 			disallow_empty = 1;
 
-			// rfc7540,l,3060,3071
-			if (((len > 0 && *b != '/') ||
-			    (len > 1 && *(b+1) == '/')) &&
-			    (strncmp(b, "*", len) != 0)) {
+			// rfc9113,l,2693,2705
+			if (len > 0 && *b != '/' &&
+			    strncmp(b, "*", len) != 0) {
 				VSLb(hp->vsl, SLT_BogoHeader,
 				    "Illegal :path pseudo-header %.*s",
 				    (int)len, b);

bin/varnishtest/tests/a02027.vtc

@@ -22,8 +22,8 @@ client c1 {
 client c1 {
 	stream 1 {
 		txreq -noadd -hdr ":authority" "foo.com" -hdr ":path" "//foo" -hdr ":scheme" "http" -hdr ":method" "GET"
-		rxrst
-		expect rst.err == PROTOCOL_ERROR
+		rxresp
+		expect resp.status == 200
 	} -run
 
 } -run