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 know we are doing this, but having interfaces for helper classes seems rather unnecessary to me. You can just type hint a class which will still allow you to pass a subclass of that.