bug#66283: 30.0.50; which-function-mode: When configured to display in header, and toggling off, then does not remove header

[Eli Zaretskii]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Cc: Mekeor Melire <mekeor@posteo.de>,  Author <sbaugh@catern.com>,
>    66283@debbugs.gnu.org,  Committer <eliz@gnu.org>
> Date: Wed, 04 Oct 2023 11:13:06 -0400
> 
> Two fixes to the previous patch:
> 
> - There was always a bug with which-function-mode where if you enabled
>   it when there were already existing buffers, it would behave
>   differently than if you enabled it and then created those buffers.
>   Namely it would enable which-func-mode for every buffer, even if the
>   buffer didn't support imenu.  This is especially noticeable when
>   toggling which-function-mode and using the header line, so I fixed it.
> 
> - Also, I accidentally dropped a paren before submitting.  Now that's
>   fixed :)

Thanks, but how about some tests for these fixes?  It's high time
which-mode had some test suite, I think.