fix: automatically_fetch_payment_terms resets given Posting Date

[bosue]

automatically_fetch_payment_terms resets a given (historical) Posting Date to today so validation of the Due Date will inevitably fail.

In test_02_so_to_deliver:
transaction_date = "2021-06-01"

In create_sales_invoice:
sinv.posting_date = so.transaction_date

Result:

due_date = datetime.date(2021, 6, 1)
posting_date = '2023-11-19'
frappe.exceptions.validationerror: Due Date cannot be before Posting / Supplier Invoice Date

Let‘s start with a failing test.

[bosue]

Closed as expected behaviour.