carddav/server: set ETag and Last-Modified if available

Some clients (e.g. Evolution) will not work properly without this. It is
up to the underlying backend to actually provide this data, the headers
will only be set if it is available.

carddav/server.go

@@ -262,7 +262,12 @@ func (b *backend) HeadGet(w http.ResponseWriter, r *http.Request) error {
 	}
 
 	w.Header().Set("Content-Type", vcard.MIMEType)
-	// TODO: set ETag, Last-Modified
+	if ao.ETag != "" {
+		w.Header().Set("ETag", internal.ETag(ao.ETag).String())
+	}
+	if !ao.ModTime.IsZero() {
+		w.Header().Set("Last-Modified", ao.ModTime.UTC().Format(http.TimeFormat))
+	}
 
 	if r.Method != http.MethodHead {
 		return vcard.NewEncoder(w).Encode(ao.Card)