DocumentCurrencyCode in Invoice, CreditNote, SelfBilledInvoice and SelfBilledCreditNote

[David Goodenough <david.goodenough@broadwellmanor.co.uk>]

I recently noticed that in Invoice, CreditNote and SelfBilledIinvoice, 
DocumentCurrencyCode is optional, but in SelfBilledCreditNote it is required.

This seems inconsistent.  

I realise that backward compatibility is necessary, so I suppose the only 
thing that can be done is to make SelfBilledCreditNote have it as optional as 
well.

As all currency amounts are recorded in Amount objects which include a 
currencyId (a mandatory attribute) actually this field is probably redundant.