code: def get_header_url(response, header_name):
      
      url = response.headers.get(header_name)
      try:
          _validate(url)
      except ValueError:
          return None
      else:
          return url
docs: Get a URL from a header requests.
  
      :param requests.Response response: REST call response.
      :param str header_name: Header name.
      :returns: URL if not None AND valid, None otherwise