[FIX] purchase: anglosaxon reconciliation test: set proper date to generated credit note

The date of the invoice is used for currency conversion in anglosaxon stock valuation. Before this fix, the tests crashed between june 6th and december 31th, as the demo data define a currency rate for USD on june 6th. Using the proper date on the credit note ensures the good rate (the one in application when making the credit) is always applied.