src/Guzzle/Http/Message/Header/HeaderCollection.php
public function add(HeaderInterface $header)
    {
        $this->headers[strtolower($header->getName())] = $header;

        return $this;
    }
public function add(HeaderInterface $header)
    {
        $this->headers[strtolower($header->getName())] = $header;

        return $this;
    }
public function add ( HeaderInterface $ header ) { $ this -> headers [ strtolower ( $ header -> getName ( ) ) ] = $ header ; return $ this ; }
Set a header on the collection

@param HeaderInterface $header Header to add

@return self
Set a header on the collection