Setting 'payment completed' for pay-later membership gives wrong start date

I think this is closely related to #15735 which was resolved for 4.6

Here is scenario

New Pay Later membership is entered on Mar 1.

Payment via bank transfer is made on Mar 1.

On March 20th admin goes in to 'edit' the contribution to record its receipt. they ensure that Mar 1 is showing as 'date received'.

Upon saving the Membership Start Date is changed to March 20th.

I think the outcome should be that the Start Date is Mar 1.

tested on demo to ensure problem (if we agree that is what it is) is still happening