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 added a workaround around this PHP limitation, please review https://github.com/owncloud/core/commit/a870895d65340254f6e962d36b92ae7b79d9d4e1 - thanks!