Get the content of a URL and return a file-like object.
    back_off=True provides retry

def download_url(url, back_off=True, **kwargs):
    """
    Get the content of a URL and return a file-like object.
    back_off=True provides retry
    """
    if back_off:
        return _download_with_backoff(url, as_file=True, **kwargs)
    else:
        return _download_without_backoff(url, as_file=True, **kwargs)