Membership pay later: extra year added on contribution completion

Ref issue #12221.
See also forum post: http://forum.civicrm.org/index.php/topic,29093.0.html - includes screenshots of the issue.

1. Create a membership type of: annual, fixed or rolling.

2. Build a contribution page for that membership where payment is "pay later" by default.

3. Submit the form to purchase a membership.
Set up "status override" to "Pending", remove "received" date from "Membership payment & receipt".

4. Open the membership record to and edit the contribution, which is currently in "pending" status.
Set Status Override to "Current" so that they can use the site while payment is being processed, then save the record before going to the next step.

5. Having pressed save, edit the associated contribution, select the payment method and update the contribution status to "Complete".

6. The membership extends itself by an extra year.