fix: let Mercado Pago take the card issuer from the card number

Paying with Mercado Pago's own test card failed with 10111 ("the issuer
does not have the BIN configured"): the issuer_id suggested by the card
form did not match the card. issuer_id is optional, and without it Mercado
Pago resolves the issuer from the BIN.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_mercadopago.py

@@ -140,6 +140,9 @@ class MercadoPagoTests(unittest.TestCase):
         self.assertNotEqual(self.requests[-1].headers['x-idempotency-key'], first_key)
         self.assertTrue(first_key.startswith('dtf-quote-q-card-'))
         self.assertEqual(body['three_d_secure_mode'], 'optional')
+        self.mp.create('q', 1000, {'mail': 'a@example.test', 'cnpj': '11222333000181'},
+                       {'type': 'card', 'token': 'tok_iss', 'payment_method_id': 'master', 'issuer_id': '24'})
+        self.assertNotIn('issuer_id', json.loads(self.requests[-1].content))
         self.assertIsNone(self.mp.create('q', 1000, {'mail': 'a@example.test', 'cnpj': '11222333000181'},
                                          {'type': 'card', 'token': 'tok_ghi', 'payment_method_id': 'visa'})['challenge'])