mod_storage_memory: Fix saving of remaining items during deletion

All data is compiled into functions that return new copies in order to
prevent mutation of stored data.

This has downsides.

Also an ugly hack.

plugins/mod_storage_memory.lua

@@ -128,7 +128,7 @@ function archive_store:delete(username, query)
 		i = old[i];
 		t = i.when;
 		if not(qstart >= t and qend <= t and (not qwith or i.with == qwith)) then
-			self:append(username, i.key, i.value, t, i.with);
+			self:append(username, i.key, i.value(), t, i.with);
 		end
 	end
 	if #new == 0 then