[BUG] Custom headers in `notmuch-message-headers` are broken

[Amin Bandali]

I got bitten by this today.

I only had a brief look at the format_headers_sprinter function
in notmuch-show.c.  Would you, David, or anyone else be able to
point out if the following makes sense, for generalizing
format_headers_sprinter to handle any arbitrary headers?

I saw this bit near the bottom of that function:

--8<---------------cut here---------------start------------->8---
if (reply) {
	sp->map_key (sp, "In-reply-to");
	sp->string (sp, g_mime_object_get_header (GMIME_OBJECT (message), "In-reply-to"));

	sp->map_key (sp, "References");
	sp->string (sp, g_mime_object_get_header (GMIME_OBJECT (message), "References"));
}
--8<---------------cut here---------------end--------------->8---

So I had a look at GMimeObject's docs on GNOME.org [0], and saw
g_mime_object_get_header_list, which returns a GMimeHeaderList*
list of headers [1], which seems to be what we're looking for.

>From there, we'd walk over 0..(g_mime_header_list_get_count-1)
indices and use g_mime_header_list_get_header_at to get each
header, and pass it to g_mime_header_get_name to get the name
which we'll pass to sp->map_key and also use to get its value
from g_mime_object_get_header.

Does that make sense?

[0]: https://developer.gnome.org/gmime/stable/GMimeObject.html
[1]: https://developer.gnome.org/gmime/stable/GMimeHeaderList.html

-amin