':param base_url: Something like http://target.com/
:return: An HTTPResponse for GET http://target.com/~_w_3_a_f_/'

def _create_non_existent_signature(self, base_url):

    headers = Headers([('Referer', base_url.url_string)])
    non_existent_user = '~_w_3_a_f_/'
    test_url = base_url.url_join(non_existent_user)
    try:
        response = self._uri_opener.GET(test_url, cache=True, headers=headers)
    except:
        msg = 'user_dir    failed    to    create    a    non    existent    signature.'
        raise BaseFrameworkException(msg)
    response_body = response.get_body()
    return response_body.replace(non_existent_user, '')
