url
  http://example.com/index.html
expected
  abcdefghijklmnopqrstuvwxyz1234567890abcdef
new_args
  proxy: http://foo:bar@proxy.example.com/

----------
GET http://example.com/index.html HTTP/1.1
Host: example.com
Connection: close
User-Agent: HTTP-Tiny/VERSION
Proxy-Authorization: Basic Zm9vOmJhcg==

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

abcdefghijklmnopqrstuvwxyz1234567890abcdef