bug#4148: mode: for a minor mode should turn on, not toggle the mode

[Chong Yidong]

Dan Nicolaescu <dann@ics.uci.edu> writes:

> Thanks, it seems to work fine.
>
> After this gets checked in, we can re-add (mode . bug-reference) to
> emacs/.dir-locals.el
>
> Any chance this patch can get onto the next release branch?

I'm having trouble coming up with a test-case for testing this patch.
Do you have one?