ensure does not properly support "classic" confinement.

[dstathis]

When a charm is already installed, and you refresh to a classic revision using ensure(state=snap_lib.SnapState.Present, revision=N, classic=True), the lib does not add --classic to the command and so it fails.

[benhoyt]

Thanks for the report. Yes, the code definitely looks a bit buggy on that front. Do you have time to put up a fix PR? If not, we can take a look later this week.