[DRE-maint] Bug#991662: schleuder: x-add-key fails for attached, binary key-material

[Georg Faerber]

Package: schleuder
Version: 3.6.0-2
Severity: important
Forwarded: https://0xacab.org/schleuder/schleuder/-/issues/495

x-add-key fails for attached, binary key material, and Schleuder tells
'no keys could be found'.

This was fixed upstream via [1], but pulling this patch into the current
version, 3.6.0-2, leads to a spec error which tests attached
quoted-printable key-material (as produced by Thunderbird).

It's currently unclear what causes this.