This is specifically to fix it for the answers file.  Anything I put in my answers file gets stringified, so I seem unable to set this to `undef` itself, though if I used a .pp file to load the class, I could easily do `passenger_scl => undef`.

So supporting 'UNSET' and 'undef' were just a couple of possibilities for users to actually unset it if they were using an answers file.