preloading has_one :through on a belongs_to

I ran across a small bug in the preloading code for has_one :through - see
http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/1507
for more details.

+1s and/or feedback for the patch (against 2-2-stable) appreciated.

--Matt Jones
al2o3cr@gmail.com