Added support for multiple chained proxies that append a comma and their hostnames to the previous X-Forwarded-Host header.
    If we have Client <---> Fwd Proxy1 (first.proxy) <---> Fwd Proxy2 (second.proxy )<---> Application (third.server), then we
    will have these values for a script executed on third.server::

    $_SERVER['HTTP_X_FORWARDED_HOST'] = 'first.proxy, second.proxy';
    $_SERVER['HTTP_HOST'] = 'third.server';

    If we use the raw value from $_SERVER['HTTP_X_FORWARDED_HOST'] for composing return URLs, we will generate invalid return URLs,
    in our case 'http://first.proxy, second.proxy/fb_oauth.php', and get a 'Oauth exception 191' or some other error message.

    If we properly process the X-Forwarded-Host value by taking the leftmost host, we will not get errors.