TaxID: Correctly set country for non 'eu_vat' taxIDs

'au_abn' and 'nz_gst' are numbers with only digits, they don't contain their
country code.
We can only guess the country from 'eu_vat' taxIDs, so let's adapt the code to
better handle AU and NZ taxIDs number.

See [the Stripe documentation] for the exact format of these two taxIDs number.

[the Stripe documentation]: https://docs.stripe.com/billing/customer/tax-ids

Author: feliixx

localstripe/resources.py

@@ -3276,7 +3276,13 @@ def __init__(self, country=None, customer=None, type=None, value=None,
             assert type in ('eu_vat', 'nz_gst', 'au_abn')
             assert _type(value) is str and len(value) > 10
             if country is None:
-                country = value[0:2]
+                if type == 'eu_vat':
+                    country = value[0:2]
+                elif type in ('nz_gst', 'au_abn'):
+                    country = type[0:2].upper()
+                else:
+                    # shouldn't happen because type is checked above
+                    assert False
             assert _type(country) is str
         except AssertionError:
             raise UserError(400, 'Bad request')