dlna: cds: use modification time as date in dlna metadata

We havn't been outputting anything for this until now, which leads to my
Samsung showing an epoch/1970 date for all files.

cmd/serve/dlna/cds.go

@@ -104,6 +104,7 @@ func (cds *contentDirectoryService) cdsObjectToUpnpavObject(cdsObject object, fi
 
 	obj.Class = "object.item." + mediaType[1] + "Item"
 	obj.Title = fileInfo.Name()
+	obj.Date = upnpav.Timestamp{Time: fileInfo.ModTime()}
 
 	item := upnpav.Item{
 		Object: obj,

cmd/serve/dlna/dlna_test.go

@@ -124,6 +124,8 @@ func TestContentDirectoryBrowseMetadata(t *testing.T) {
 	require.NotContains(t, string(body), html.EscapeString("<item "))
 	// if there is a childCount, it better not be zero
 	require.NotContains(t, string(body), html.EscapeString(" childCount=\"0\""))
+	// should have a dc:date element
+	require.Contains(t, string(body), html.EscapeString("<dc:date>"))
 }
 
 // Check that the X_MS_MediaReceiverRegistrar is faked out properly.