url
  http://example.com/cb.html
expected
  abcdefghijklmnopqrstuvwxyz1234567890abcdef
data_cb
  sub { $main::data .= $_[0] }
----------
GET /cb.html HTTP/1.1
Host: example.com
Connection: close
User-Agent: HTTP-Tiny/VERSION

----------
HTTP/1.1 200 OK
Date: Thu, 03 Feb 1994 00:00:00 GMT
Content-Type: text/plain
Content-Length: 44

abcdefghijklmnopqrstuvwxyz1234567890abcdef