url
  http://example.com/modified.txt
headers
  if-modified-since: Tue, 01 Feb 1994 12:00:00 GMT
----------
GET /modified.txt HTTP/1.1
Host: example.com
Connection: close
If-Modified-Since: Tue, 01 Feb 1994 12:00:00 GMT
User-Agent: HTTP-Tiny/VERSION

----------
HTTP/1.1 200 OK
Date: Thu, 03 Feb 1994 00:00:00 GMT
Last-Modified: Wed, 02 Feb 1994 00:00:00 GMT
Content-Type: text/plain
Content-Length: 42

abcdefghijklmnopqrstuvwxyz1234567890abcdef

