CFO-1710 - Update eSLAP spec to only support ISO 8601(YYYY-MM-DD)

Update invoice spec to only support ISO 8601(YYYY-MM-DD) date format
Merge pull request #5 from C2FO/cfo-1710

CFO-1710 - Update eSLAP spec to only support ISO 8601(YYYY-MM-DD)