Membership Contribution with price set doesn't update existing, expired membership

When users make a membership contribution via an online contribution page that uses a price set, and they have an existing membership that is past its membership end date, the existing membership is not updated.

More details are at http://forum.civicrm.org/index.php/topic,19191.0.html - here is the short synopsis:

• It happens when an existing but expired member enters a membership renewal via the contribution page with a price set (might happen in other circumstances, such as a contribution page without a price set, but the price set example is the one I have tested).

• It also happens with test contributions, so you can test easily

• It happens whenever the end date for the membership you are trying to renew is before the current date.

• The problems appears to be caused by the invocation of $transaction = new CRM_Core_Transaction(); in CRM_Member_BAO_Membership::create (about line 320). If that line and other references to $transaction are removed everything seems to work AOK--both the membership and contribution are recorded successfully.