patch for #5488

Hi,

Is there anybody can review my little patch for
https://rails.lighthouseapp.com/projects/8994/tickets/5488-default_scope-doesnt-work-at-a-late-point
? It fix the problem that default_scope doesn't merge two where
clauses.

Rohit has tested the patch (thanks!) but there's no updates after
that ...

Thanks,
Jan

I've applied it. Thanks!