fix: static cache-control on /ipfs dir listing

just a precaution, since we still have TTL=0 for many DNSLink responses
due to #329 (comment)

gateway/gateway_test.go

@@ -130,9 +130,9 @@ func TestHeaders(t *testing.T) {
 			path         string
 			cacheControl string
 		}{
-			{"/ipns/example.net/", "public, max-age=30"},                 // As generated directory listing
-			{"/ipns/example.com/", "public, max-age=55"},                 // As generated directory listing (different)
-			{"/ipns/unknown.com/", ""},                                   // As generated directory listing (unknown)
+			{"/ipns/example.net/", "public, max-age=30, stale-while-revalidate=2678400"}, // As generated directory listing
+			{"/ipns/example.com/", "public, max-age=55, stale-while-revalidate=2678400"}, // As generated directory listing (different)
+			{"/ipns/unknown.com/", ""},                                   // As generated directory listing (unknown TTL)
 			{"/ipns/example.net/foo/", "public, max-age=30"},             // As index.html directory listing
 			{"/ipns/example.net/foo/index.html", "public, max-age=30"},   // As deserialized UnixFS file
 			{"/ipns/example.net/?format=raw", "public, max-age=30"},      // As Raw block

gateway/handler_unixfs_dir.go

@@ -140,7 +140,7 @@ func (i *handler) serveDirectory(ctx context.Context, w http.ResponseWriter, r *
 	if rq.ttl > 0 {
 		// Use known TTL from IPNS Record or DNSLink TXT Record
 		w.Header().Set("Cache-Control", fmt.Sprintf("public, max-age=%d, stale-while-revalidate=2678400", int(rq.ttl.Seconds())))
-	} else {
+	} else if !rq.contentPath.Mutable() {
 		// Cache for 1 week, serve stale cache for up to a month
 		// (style of generated HTML may change, should not be cached forever)
 		w.Header().Set("Cache-Control", "public, max-age=604800, stale-while-revalidate=2678400")