Confirmation of pending payment incorrectly alters membership join date

We are seeing a behaviour in 2.0.x and 2.2.3 where a CiviContribute join or renew form will lose membership join dates as a member renews, if they select "Pay later by check". The join date is correct after the renewal, but is changed to the start date when the pending status is updated by the administrator.

To reproduce on demo:

0. Configure a CiviContribute renewal page with Pay Later enabled

1. Configure a membership for Tester McTester with join_date and start_date = 2008

2. Renew the membership using the CiviContribute page, using "Pay later by check" to set payment = pending

3. Check membership details

4. Find membership pending as admin, click "edit" for pending membership

5. Change payment status from Pending to Completed. When you click Save on this form, the join date for the membership will be set incorrectly.

6. Verify that the join date is now incorrect for this membership.

Example with links and images at http://forum.civicrm.org/index.php/topic,8136.0.html

Confirmed in 2.0.x and 2.2.3