cmd/noxy: add a minimal unconditional cache-control header

so the browsers will hopefully cache responses for 1h and reduce load on
the server.

a future version will propagate cache-control from upstream.
part of #3

cmd/noxy/main.go

@@ -24,6 +24,7 @@ var (
 	listenAddr       = flag.String("addr", "127.0.0.1:8889", "listen address")
 	cacheDir         = flag.String("cachedir", "/tmp", "absolute cache dir")
 	maxFileSize      = flag.Int64("maxfilesize", 1<<23, "refuse to handle files larger than this, in bytes")
+	minCacheAge      = flag.Uint("mincacheage", 3600, "min cache-control max-age value, in seconds")
 	idleRelayTimeout = flag.Duration("idlerelaytimeout", 10*time.Minute, "remove relay connections after idling this long")
 	showVersion      = flag.Bool("V", false, "print version and exit")
 
@@ -164,6 +165,7 @@ func handleMeta(w http.ResponseWriter, r *http.Request) {
 		Images: meta.ImageURLs,
 	}
 	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("Cache-Control", fmt.Sprintf("max-age=%d", *minCacheAge))
 	json.NewEncoder(w).Encode(res)
 }
 
@@ -183,6 +185,7 @@ func handleData(w http.ResponseWriter, r *http.Request) {
 	}
 	defer ds.Close()
 	w.Header().Set("Content-Type", ds.ContentType)
+	w.Header().Set("Cache-Control", fmt.Sprintf("max-age=%d", *minCacheAge))
 	io.Copy(w, ds)
 }