Fix incorrect backfilling

This would have the effect that item TTL is continually reset as long as an item is returned by fetch

core/items.go

@@ -58,7 +58,7 @@ func BackfillItem(new *Item, old *Item) {
 		new.Title = old.Title
 	}
 	if new.Ttl == 0 {
-		new.Tts = old.Tts
+		new.Ttl = old.Ttl
 	}
 	if new.Ttd == 0 {
 		new.Ttd = old.Ttd