message: Add utf-16 and utf-32 encoded test-cases to jwt_validation_test.py.
  says that only encoding should be accepted. So I think it's best to add a test for that.
diff: @@ -499,6 +499,24 @@ class JwtTest(parameterized.TestCase):
  with self.assertRaises(tink.TinkError):
  jwt_mac.verify_mac_and_decode(token, EMPTY_VALIDATOR)
  + @parameterized.parameters(SUPPORTED_LANGUAGES)
  + def test_verify_token_with_utf_16_encoded_payload_fails(self, lang):
  + token = generate_token_from_bytes('{"alg":"HS256"}'.encode('utf-8'),
  + '{"iss":"joe"}'.encode('utf-16'))
  + jwt_mac = testing_servers.jwt_mac(lang, KEYSET)
  +
  + with self.assertRaises(tink.TinkError):
  + jwt_mac.verify_mac_and_decode(token, EMPTY_VALIDATOR)
  +
  + @parameterized.parameters(SUPPORTED_LANGUAGES)
  + def test_verify_token_with_utf_32_encoded_payload_fails(self, lang):
  + token = generate_token_from_bytes('{"alg":"HS256"}'.encode('utf-8'),
  + '{"iss":"joe"}'.encode('utf-32'))
  + jwt_mac = testing_servers.jwt_mac(lang, KEYSET)
  +
  + with self.assertRaises(tink.TinkError):
  + jwt_mac.verify_mac_and_decode(token, EMPTY_VALIDATOR)
  +
  @parameterized.parameters(SUPPORTED_LANGUAGES)
  def test_verify_token_with_many_recursions(self, lang):
  num_recursions = 10
  