Upgrade fixes

- Connect changed the API slightly, it does not return whole response object (like `Net::HTTPServerError` but only the error code number).
- some small fixes included
No, I added code to handle it globally at `SUSE::Connect::Credentials` class to avoid logging password globally (see https://github.com/SUSE/connect/blob/master/lib/suse/connect/credentials.rb#L55).