Do only follow HTTP and HTTPS redirects

We do not want to follow redirects to other protocols since they might allow an adversary to bypass network restrictions. (i.e. a redirect to ftp:// might be used to access files of a FTP server which might be in a secure zone and not be reachable from the net but from the ownCloud server)
I wrote a smaller wrapper for this kind of *beep*, that might be useful. See https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/phpbb/php/ini.php