Description:
------------
If there is any header in the http request that contains the string 'host:' in their value before the actual host header the requests ends up being sent with two headers Host, the one specified by the user and the default one that's injected in case the header wasn't present.

Test script:
---------------
<?php
$opts = array(
  'http'=>array(
    'method'=>"GET",
    'header'=>"RandomHeader: localhost:8080\r\n" .
              "Cookie: foo=bar\r\n" .
              "Host: testcustomheader  \r\n"
  )
);
$context = stream_context_create($opts);
$fp = fopen('http://myserver.com', 'r', false, $context);
fpassthru($fp);

Expected result:
----------------
The raw headers should only contain the host header specified by the user that was sent in the context.

Actual result:
--------------
We get a faulty request with two Host headers instead:

GET / HTTP/1.0
Host: myserver.com
RandomHeader: localhost:8080
Cookie: foo=bar
Host: testcustomhostheader