autosave for has_one does not work properly with primary_key option

Running into problems with :primary key option and autosave association - it sets it back to the id column of the parent instance when it shouldn't be saving it at all - it also sets it incorrectly

using the models from the test suite

f = Firm.new(:name => 'Firm with account_using_primary_key', :firm_id => 2)
a = Account.create(:credit_limit => 20, :firm_id => f.firm_id)

assoc = f.account_using_primary_key(true)
f.save

this code sets a.firm_id = f.id and then saves when it should not save - the test I wrote is pretty crummy - I just freeze the Account and show that it bails when trying to save the Firm - but will show the problem where it is saving when it should not

I also attached a passing patch which should do both things - alter the check if it should save - and set the proper value which is :primary_key if set, id if not