[BUG] membership_delegated_partner: changing delegated member

[astirpe]

Changing delegated member in invoice doesn't update correctly the membership lines.

Tested in V12, probably also occurs on previous versions.

Steps to reproduce:

• Enable group "Delegated partner in membership" for current user.
• Create a Membership Product with Duration that includes the date of today.
• Create an invoice with the above Membership Product. Add a delegated member. Save.
• A line is created in the Membership lines of the selected delegated member. OK.
• No line is present in the Membership lines of the customer. OK.
• Edit the invoice. Remove the delegated member. Save.

Result:

• The membership is NOT updated: no new line is created in the Membership lines of the customer. NOT OK.
• The membership is still present in the Membership lines of the delegated member. NOT OK.

Expected:

• The membership must be updated, a new line must be created in the members lines of the customer.
• No membership line must be present in the members lines of the delegated member.

[astirpe]

I'm proposing a fix in #79