The "Save to sent items" value is not saved

MailSettingsForm::submitForm() uses the following code.

    parent::submitForm($form, $form_state);

    $this->config('graph_mail.settings')
      ->set('tenant_id', $form_state->getValue('tenant_id'))
      ->set('client_id', $form_state->getValue('client_id'))
      ->set('client_secret', $form_state->getValue('client_secret'))
      ->set('user_id', $form_state->getValue('user_id'))
      ->set('version', $form_state->getValue('version'))
      ->set('default_mail', $form_state->getValue('default_mail'))
      ->save();

It does not save the value for Save to sent items, which is submitted in the $form['save_to_sent_items'] form element.

The same bug is present in the 1.0.x branch.