def fetch_http_fallback_identifier(head_request)
      if head_request.key?('last-modified')
        head_request['last-modified']
      elsif head_request.key?('content-length')
        head_request['content-length']
      else
        Time.now.to_s
      end
    end
def fetch_http_fallback_identifier(head_request)
      if head_request.key?('last-modified')
        head_request['last-modified']
      elsif head_request.key?('content-length')
        head_request['content-length']
      else
        Time.now.to_s
      end
    end
Find a "next best" ID when the HEAD request does not return an "ETag"
 header.