server: fix "multiple response.WriteHeader calls".

The server no does nothing for errors other than ErrNotFound (for which
it calls http.NotFound). This is becase when getting error on writing,
we already wrote headers, so can't respond with status 500.

Possibly log errors in future?

server.go

@@ -43,10 +43,6 @@ func (h *captchaHandler) serve(w http.ResponseWriter, id, ext string, download b
 		}
 		return WriteImage(w, id, h.imgWidth, h.imgHeight)
 	case ".wav":
-		if !download {
-			w.Header().Set("Content-Type", "audio/x-wav")
-		}
-		//return WriteAudio(w, id)
 		//XXX(dchest) Workaround for Chrome: it wants content-length,
 		//or else will start playing NOT from the beginning.
 		//Filed issue: http://code.google.com/p/chromium/issues/detail?id=80565
@@ -55,6 +51,9 @@ func (h *captchaHandler) serve(w http.ResponseWriter, id, ext string, download b
 			return ErrNotFound
 		}
 		a := NewAudio(d)
+		if !download {
+			w.Header().Set("Content-Type", "audio/x-wav")
+		}
 		w.Header().Set("Content-Length", strconv.Itoa(a.EncodedLen()))
 		_, err := a.WriteTo(w)
 		return err
@@ -74,11 +73,8 @@ func (h *captchaHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		Reload(id)
 	}
 	download := path.Base(dir) == "download"
-	if err := h.serve(w, id, ext, download); err != nil {
-		if err == ErrNotFound {
-			http.NotFound(w, r)
-			return
-		}
-		http.Error(w, "error serving captcha", http.StatusInternalServerError)
+	if h.serve(w, id, ext, download) == ErrNotFound {
+		http.NotFound(w, r)
 	}
+	// Ignore other errors.
 }