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)
This might actually also need some handling for when a redirect does not end with a final HTTP(S) location. getFinalLocationOfURL() probably has to give that as feedback.