CUST-932 Add message/rfc822-headers as an rfc header

[tiger-zz]

Background

We were failing to parse a bounce message that had this sub-part MIME:

// some headers ommitted for brevity
Return-Path: <>
Date: Tue, 07 Jun 2022 13:04:20 +0200
From: "Mail Delivery System" <[email protected]>
To: [email protected]
Subject: Mail delivery failed: returning message to sender
Auto-Submitted: auto-replied
Content-Type: multipart/report;boundary="4zbut9xw88wjbg96ad6j2jsqofz0dm82mspkaumnz24trpanhmdj56iegc03veyln1s";
 report-type="delivery-status"
MIME-Version: 1.0
X-UI-Out-Filterresults: unknown:0;V03:K0:lz0CEy2Glmc=:T1Qr3hOrYeek9s2b714PbA
 ==
X-Spam-Flag: NO

--4zbut9xw88wjbg96ad6j2jsqofz0dm82mspkaumnz24trpanhmdj56iegc03veyln1s
// a few other mimeparts

--4zbut9xw88wjbg96ad6j2jsqofz0dm82mspkaumnz24trpanhmdj56iegc03veyln1s
Content-Type: message/rfc822-headers
Content-Transfer-Encoding: 7bit

//Some headers ommitted
Date: Tue, 7 Jun 2022 11:04:19 +0000
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
X-Spam-Flag: YES
X-Spam-Flag: YES

--4zbut9xw88wjbg96ad6j2jsqofz0dm82mspkaumnz24trpanhmdj56iegc03veyln1s--

Only the overall MIME multipart MIME part and the erronous MIME part were included. The existing flanker library did not treat the Content-Type header in the message/rfc822-headers section correctly as it did not properly categorize the Content-Type of message/rfc822-headers as a container

Tests

Made the change to my local instance of flanker, than ran our parsing logic against the failing mime. This parsed the mime correctly

I could not get the nosetests to run correctly on my local machine

[pengfeiye]

should this not be self == 'text/rfc822-headers' or self == 'message/rfc822-headers'? this method is supposed to return a bool but instead, you are trying to return the 'message/rfc822-headers' string when self == 'text/rfc822-headers' condition is not met 🤔