url
  http://example.com/not-modified.txt
----------
GET /not-modified.txt HTTP/1.1
Host: example.com
Connection: close
If-Modified-Since: Tue, 01 Feb 1994 00:00:00 GMT
User-Agent: HTTP-Tiny/VERSION

----------
HTTP/1.1 304 Not Modified
Date: Thu, 03 Feb 1994 00:00:00 GMT
Last-Modified: Tue, 01 Feb 1994 00:00:00 GMT