Add a way to download only the full headers and not the rest of the content

Expectead behavior

Print complete header when pressing Show headers. I don't care about highlighting, plaintext is okay but print the complete header.

Actual behavior

k9 is not showing the full header. It appears as if just some special/known(?) parts of the header are chosen.

Example

This is a classical spam-header -- hence I leave the senders information.

k9

Message-ID: <ovyiwnq35671246.04083004@mail.mtsbu.us>
From: Zusatzliches Einkommen <ovyiwnq@mtsbu.us>
To: <censored@censored.com>
Subject: Zusätzliches Einkommen für alle
Date: Mon, 25 Sep 2017 23:41:23 +0300
MIME-Version: 1.0
Content-Type: multipart/related; boundary="----9AVLIOCRAVG8Z8SJ6PZ1Z962WKLRFS"
Content-Transfer-Encoding: 7bit

Actually / Expected

Received: from mailer.censored.com (censored.10.17) by um-excht-a01.um.censored.com
 (censored.11.221) with Microsoft SMTP Server id 14.3.361.1; Mon, 25 Sep 2017
 22:41:36 +0200
Received: from censored.censored.com ([censored.82.140])    by mailer.censored.com
 with esmtps (TLSv1:DHE-RSA-AES256-SHA:256) (Exim 4.80) (envelope-from
 <ovyiwnq@mtsbu.us>)    id 1dwaC8-0001bk-Oc; Mon, 25 Sep 2017 22:41:36 +0200
Received: by censored.censored.com (Postfix)    id 591195A7; Mon, 25 Sep 2017
 22:41:36 +0200 (CEST)
Delivered-To: censored@censored.censored.com
Received: by censored.censored.com (Postfix, from userid 65534) id
 41631545C; Mon, 25 Sep 2017 22:41:36 +0200 (CEST)
X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on
    censored.censored.com
X-Spam-Level:
X-Spam-Status: No, score=-3.9 required=5.0 tests=BAYES_00,EXTRA_MPART_TYPE,
    HTML_IMAGE_ONLY_04,HTML_MESSAGE,HTML_SHORT_LINK_IMG_1,MPART_ALT_DIFF,
    RCVD_IN_DNSWL_HI,TVD_SPACE_RATIO,T_AXB_MIME_IMG830 autolearn=ham version=3.3.2
Received: from mailer.censored.com (mailer.censored.com [censored.10.26])   by  
 censored.censored.com (Postfix) with ESMTPS id A9D1A5A7;   Mon, 25 Sep 2017
 22:41:29 +0200 (CEST)
Received: from mfilter-123-1-1.mx.srv.dfn.de ([194.95.232.60])  by  
 mailer.censored.com with esmtps (TLSv1.2:DHE-RSA-AES256-GCM-SHA384:256)    (Exim
 4.80)  (envelope-from <ovyiwnq@mtsbu.us>)  id 1dwaC1-0001aM-J4; Mon, 25 Sep 
 2017 22:41:29 +0200
Received: from mail.mtsbu.us (mail.mtsbu.us [89.163.212.18])    by  
 mfilter-123-1-1.mx.srv.dfn.de (Postfix) with ESMTP;    Mon, 25 Sep 2017 22:41:25
 +0200 (CEST)
Received: from mtsbu.us (mail.mtsbu.us [89.163.212.18]) by mail.mtsbu.us
 (Postfix) with ESMTPA id D21A11C0F38;  Mon, 25 Sep 2017 23:41:21 +0300 (EEST)
Message-ID: <ovyiwnq35671246.04083004@mail.mtsbu.us>
From: Zusatzliches Einkommen <ovyiwnq@mtsbu.us>
To: <censored@censored.com>
Subject: =?utf-8?B?WnVzw6R0emxpY2hlcyBFaW5rb21tZW4gZsO8ciBhbGxl?=
Date: Mon, 25 Sep 2017 23:41:23 +0300
Content-Type: multipart/related; type="multipart/alternative";
    boundary="----=_NextPart_000_0006_01D33655.5FBCB8C0"
Precedence: bulk
List-Id: b83348430v73546086
X-Complaints-To: abuse@mtsbu.us
List-Unsubscribe: <http://mtsbu.us/ru/unsubscribe/do?hash=8100057523076787>
X-Virus-Scanned: (clean) by clamav
Return-Path: ovyiwnq@mtsbu.us
X-MS-Exchange-Organization-AuthSource: UM-EXCHT-A01.um.censored.com
X-MS-Exchange-Organization-AuthAs: Anonymous
X-MS-Exchange-Organization-PRD: mtsbu.us
X-MS-Exchange-Organization-SenderIdResult: Fail
Received-SPF: Fail (UM-EXCHT-A01.um.censored.com: domain of ovyiwnq@mtsbu.us does
 not designate censored.82.140 as permitted sender)
 receiver=UM-EXCHT-A01.um.censored.com; client-ip=censored.82.140;
 helo=mailer.censored.com;
X-MS-Exchange-Organization-SCL: 5
X-MS-Exchange-Organization-PCL: 2
X-MS-Exchange-Organization-Antispam-Report: DV:3.3.16631.866;SID:SenderIDStatus Fail;OrigIP:censored.82.140
MIME-Version: 1.0

Environment

K-9 Mail version: 5.208

Android version: 7.1.2

Account type (IMAP, POP3, WebDAV/Exchange): IMAP

Start by locating the Android message-view code that handles Show headers and inspect how the displayed header fields are selected. Confirm the expected behavior from the issue example, then verify that Show headers presents the complete raw header without the message body.