fix: Ignore hidden headers in IMF section

Hidden headers are nonstandard, so they aren't DKIM-signed by e.g. OpenDKIM if they appear in IMF
section.

test-data/message/mail_with_user_and_group_avatars.eml

@@ -1,7 +1,5 @@
 Chat-Group-ID: WVnDtF5azch
 Chat-Group-Name: =?utf-8?q?testgr1?=
-Chat-Group-Avatar: group-image.png
-Chat-User-Avatar: avatar.png
 Subject: =?utf-8?q?Chat=3A_testgr1=3A_hi!_?=
 Date: Thu, 12 Dec 2019 17:24:03 +0000
 X-Mailer: Delta Chat Core 1.0.0-beta.15/CLI
@@ -14,6 +12,8 @@ Content-Type: multipart/mixed; boundary="LV8nfXkpyyn39fsVyoB1b29PKDMeb5"
 
 --LV8nfXkpyyn39fsVyoB1b29PKDMeb5
 Content-Type: text/plain; charset=utf-8
+Chat-Group-Avatar: group-image.png
+Chat-User-Avatar: avatar.png
 
 hi! 
 

test-data/message/mail_with_user_avatar.eml

@@ -1,4 +1,3 @@
-Chat-User-Avatar: avatar.png
 Subject: =?utf-8?q?Chat=3A_this_is_a_message_with_a_=2E=2E=2E?=
 Message-ID: Mr.wOBwZNbBTVt.NZpmQDwWoNk@example.org
 In-Reply-To: Mr.ETXqza5-WpB.zDEYOLECxAw@example.org
@@ -12,6 +11,7 @@ Content-Type: multipart/mixed; boundary="luTiGu6GBoVLCvTkzVtmZmwsmhkNMw"
 
 --luTiGu6GBoVLCvTkzVtmZmwsmhkNMw
 Content-Type: text/plain; charset=utf-8
+Chat-User-Avatar: avatar.png
 
 this is a message with a profile-image attached
 

test-data/message/mail_with_user_avatar_deleted.eml

@@ -1,5 +1,3 @@
-Content-Type: text/plain; charset=utf-8
-Chat-User-Avatar: 0
 Subject: =?utf-8?q?Chat=3A_profile_image_deleted?=
 Message-ID: Mr.tsgoJgn-cBf.0TkFWKJzeSp@example.org
 Date: Sun, 08 Dec 2019 23:28:30 +0000
@@ -7,8 +5,16 @@ X-Mailer: Delta Chat Core 1.0.0-beta.12/CLI
 Chat-Version: 1.0
 To: <tunis3@example>
 From: "=?utf-8?q??=" <tunis4@example.org>
+Content-Type: multipart/mixed; boundary="luTiGu6GBoVLCvTkzVtmZmwsmhkNMw"
+
+
+--luTiGu6GBoVLCvTkzVtmZmwsmhkNMw
+Content-Type: text/plain; charset=utf-8
+Chat-User-Avatar: 0
 
 profile image deleted
 
 -- 
 Sent with my Delta Chat Messenger: https://delta.chat
+
+--luTiGu6GBoVLCvTkzVtmZmwsmhkNMw--