fix occasion when _refresh_token being treated like the old attribute, as opposed to a methdo

Author: mikeedjones

google/auth/external_account_authorized_user.py

@@ -343,7 +343,7 @@ def revoke(self, request):
             request, self._refresh_token_val, "refresh_token", self._revoke_url
         )
         self.token = None
-        self._refresh_token = None
+        self._refresh_token_val = None
 
     @_helpers.copy_docstring(credentials.Credentials)
     def get_cred_info(self):

tests/test_external_account_authorized_user.py

@@ -367,7 +367,7 @@ def test_revoke_auth_success(self):
             ),
         )
         assert creds.token is None
-        assert creds._refresh_token is None
+        assert creds._refresh_token_val is None
 
     def test_revoke_without_revoke_url(self):
         request = self.make_mock_request()