nested attributes test should rely on associations to verify the results on count

currently nested attributes tests relying on assert_difference, assert_no_difference to verify results.

 assert_difference('Ship.count', 1) do
   @pirate.reload.update_attributes(:update_only_ship_attributes => { :name => 'Mayflower' })
 end

as opposed to testing the actual relations.

 @pirate.reload.update_attributes(:update_only_ship_attributes => { :name => 'Mayflower' })

 assert_not_nil @pirate.reload.ship
 assert_equal @ship, @pirate.reload.ship

this patch fixes nested attributes tests.