Use identifier code for getting vat code

aeat_mapping.py

@@ -37,7 +37,7 @@ class BaseTrytonInvoiceMapper(Model):
     def counterpart_nif(self, invoice):
         nif = ''
         if invoice.party.tax_identifier:
-            nif = invoice.party.tax_identifier
+            nif = invoice.party.tax_identifier.code
         elif invoice.party.identifiers:
             nif = invoice.party.identifiers[0].code
         if nif.startswith('ES'):

party.py

@@ -28,4 +28,4 @@ class Party(metaclass=PoolMeta):
                     return identifier.code
                 return identifier.code[2:]
             elif name == 'sii_vat_country':
-                return identifier[:2]
+                return identifier.code[:2]