The unsetHeader bad work, 
for example
$curl->setHeader('Host','example.com');
$curl->unsetHeader('Host');
$curl->get('http://127.0.0.1/');
---------
This request is make with Header "Host" = '' (empty string), because unsetHeader bad work

with the my fix the header 'Host' with be deleted and curl auto-retrive the right header.
horever is already possible make request with header 'Host' = '' (empty string) with $curl->setHeader('Host', '' );

also remove a bad comment about fuction
   *		
-     * @return string
in setHeader()