`isofs` seems to report `case_insensitive` incorrectly for Rock Ridge

From inspection (I haven't done testing) of fs.archive 0.7.2, it looks to me like `fs.archive.isofs` reports that it is `case_insensitive` in the following way:

if `self._rock_ridge or self._joliet`: `True`
else if `self._cd.interchange_level < 4`: `True`
else: `False`

I searched online and according to quite a few pages I found it seems like Rock Ridge is actually case-sensitive. I don't know if pycdlib has any restrictions of its own in this regard though.

After looking into this, I figured out that I don't actually care how it reports `case_insensitive` for my project, so I'm not actually affected by this issue.