bug#28331: [gnus] Articles loose read status on expiry

[Nikolaus Rath]

When marking an article as read and for expiry, and the expiry target is
another group, the article ends up being unread again in the target
group.

An easy fix is to take `gnus-summary-expire-articles' out of
`gnus-summary-prepare-exit-hook', and instead call it explicitly in
`gnus-summary-exit' at a later point.

However, this results in the article not just being marked read in the
target group (as intended), but also still marked for expiry (not
good). I could not figure out how to change that.

I've observed this with the nnimap backend.

Ma Gnus v0.14
GNU Emacs 25.1.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.12)
 of 2017-04-23, modified by Debian

Best,
-Nikolaus

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