txn.add_header not appending headers correctly

[msimerson]

Describe the bug

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Date: Tue, 5 Apr 2022 22:16:37 +0000
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Expected behavior

Appended headers belong in the header section, not at the end of the message body.

Observed behavior

Sometimes, appended headers aren't put in the right spot. This is atypical behavior which I see on occasion.

System Info:

Please report your OS, Node version, and Haraka version by running this shell script on your Haraka server and replacing this section with the output.

Haraka	Haraka.js — Version: 2.8.28
Node	v14.18.1
OS	FreeBSD haraka 13.0-RELEASE-p7 FreeBSD 13.0-RELEASE-p7 #0: Mon Jan 31 18:24:03 UTC 2022 root@amd64-builder.daemonology.net:/usr/obj/usr/src/amd64.amd64/sys/GENERIC amd64
openssl	OpenSSL 1.1.1h-freebsd 24 Aug 2021