GET /api/invoices: exception if due_days is a negative number

[mplattner]

I retrieve invoices from the API via the /api/invoices endpoint.

The validation (integer) for the due_days field fails and an exception is thrown if the due_days field of an invoice is a negative number in the response.
Negative due_days values are possible because there is no validation in the Salesking webinterface that disallows setting a due date prior to the invoice date.

As a workaround, I fixed it by changing the schema to "number" for the due_days field.

I think the Salesking webinterface validation should be updated, but the API validation as well for all records where due_days results in a negative now (before the webinterface change).