Only accept HTTP and HTTPS as protocol

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)

See https://github.com/owncloud/core/pull/11032 for the change in ownCloud and https://github.com/fruux/sabre-http/pull/14 for the PR for sabre-http