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)
`getFinalLocationOfURL` will return the last HTTPS or HTTP location and stop processing after that. Basically:

HTTPS -> HTTP -> FTP (second one will be returned)
HTTPS -> FTP -> HTTP (first one will be returned)
HTTPS -> HTTP -> HTTPS (last one will be returned)