Return the show feed URL with different protocol.

def show_url(context, **kwargs):
    """Return the show feed URL with different protocol."""
    if len(kwargs) != 2:
        raise TemplateSyntaxError(_('"show_url" tag takes exactly two keyword arguments.'))
    request = context['request']
    current_site = get_current_site(request)
    url = add_domain(current_site.domain, kwargs['url'])
    return re.sub(r'https?:\/\/', '%s://' % kwargs['protocol'], url)