Duedate bug in 1.2.20

[Luke <..hidden..>]

In AP, at least for vendor invoices (not yet tested on AP Transactions, or 
purchase orders, although I seem to remember this happening somewhere 
else).

If you select a vendor, and update, then remove the generated due date, it 
will not be updated or set to default for that vendor on post.

What should happen, is the same thing that does on sales invoices for 
blank due dates.

What does happen, is an insert error for a blank date type.

Luke