be a little more precise in copying headers

it doesn't matter too much right now, given the headers that are being
copied, but this now makes sure that all header values get copied over
if multiple are present.

Author: willnorris

imageproxy.go

@@ -109,21 +109,28 @@ func (p *Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	w.Header().Add("Last-Modified", resp.Header.Get("Last-Modified"))
-	w.Header().Add("Expires", resp.Header.Get("Expires"))
-	w.Header().Add("Etag", resp.Header.Get("Etag"))
+	copyHeader(w, resp, "Last-Modified")
+	copyHeader(w, resp, "Expires")
+	copyHeader(w, resp, "Etag")
 
 	if is304 := check304(r, resp); is304 {
 		w.WriteHeader(http.StatusNotModified)
 		return
 	}
 
-	w.Header().Add("Content-Length", resp.Header.Get("Content-Length"))
-	w.Header().Add("Content-Type", resp.Header.Get("Content-Type"))
+	copyHeader(w, resp, "Content-Length")
+	copyHeader(w, resp, "Content-Type")
 	defer resp.Body.Close()
 	io.Copy(w, resp.Body)
 }
 
+func copyHeader(w http.ResponseWriter, r *http.Response, header string) {
+	key := http.CanonicalHeaderKey(header)
+	if value, ok := r.Header[key]; ok {
+		w.Header()[key] = value
+	}
+}
+
 // allowed returns whether the specified URL is on the whitelist of remote hosts.
 func (p *Proxy) allowed(u *url.URL) bool {
 	if len(p.Whitelist) == 0 {