Fixed bug that prevented sitemap generation override in a Page subclass

If a developer wants to turn off urls for a specific page as documented in http://docs.wagtail.io/en/latest/contrib_components/sitemap_generation.html#customising they should be able to add:

    def get_sitemap_urls(self):
        return []

but get_urls() calls only the parent class rather than the specific child class. This bug is also mentioned in issue #833