Enable to overwrite HTTP Header for serverPost

Hi U-Zyn,

I created new strategy for Yahoo! JAPAN OAuth 2.0 implementation,   
and it needs to set additional HTTP Authorizaton Header for Access Token Request.

    Authorization: Basic (Base64 encoded Client Credentials)

I call serverPost method using options parameter in strategy file.  
[YahoojpStrategy.php](https://github.com/ritou/opauth-yahoojp/blob/master/YahoojpStrategy.php)

    $options = array(
        'http' => array(
            'header' => "Content-type: application/x-www-form-urlencoded\r\n".
                        "Authorization: Basic ".base64_encode($this->strategy['client_id'] . ':' . $this->strategy['client_secret'])
        )
    );

However, OpauthStrategy seems to overwrite the $options['http'] with default value by array_merge.  
[Current OpauthStrategy.php](https://github.com/ritou/opauth/blob/74154433b78989e2f6913e7358a826bd77aae8bb/lib/Opauth/OpauthStrategy.php)

    $stream = array_merge($options, $stream);

I changed this logic.
If $options has 'http' array, it overwrites default stream with optional value.

Thanks.
Ryo.