Yet another encoding problem

[MMaus]

When I run a GreenMail server (as Spring bean) in an application and send an utf-8 coded email with umlauts via Thunderbird, and then fetch that email again via Thunderbird, I might have the wrong encoding.
Whether or not this occurs depends on the encoding which is used for the JVM, i.e. running with -Dfile.encoding=iso-8859-1 works, but -Dfile.encoding=utf-8 does not work (contrary to what you would expect - I know).
I think I spotted the issue in the code - in GreenMailUtil#getWholeMessage treats every String as ISO-8859-1 encoded, and since it transforms byte[] into String (which is UTF-16 in Java), that's actually a conversion of data.

I'm working on a PR to fix this, if that's okay.

Below is the received Mail after my test scenario; the actual message body should be "Täst"

From - Fri Feb 18 10:28:22 2022
X-Account-Key: account4
X-UIDL: 1
X-Mozilla-Status: 0003
X-Mozilla-Status2: 00000000
X-Mozilla-Keys:
Return-Path: foo@test.server
Received: from 127.0.0.1 (HELO [127.0.0.1]); Fri Feb 18 10:28:21 CET 2022
Message-ID: bf6c9320-9c2b-2063-de0a-bfde973b481e@test.server
Date: Fri, 18 Feb 2022 10:28:21 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
Thunderbird/91.6.1
To: foo@test.server
From: "[Localhost] Moritz Test" foo@test.server
Subject: =?UTF-8?B?U3ViasOka3Qy?=
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

Täst

[marcelmay]

Thx, @MMaus and looking forward to your PR!

[MMaus]

If the PR is okay, I can of course provide the corresponding one for master.

[MMaus]

I need to re-work this PR

[MMaus]

PR is open now - I hope the 1.6.x branch is okay, of course I can provide it for master as well

[marcelmay]

PR looks good and please make PR against master.

Thanks alot!

[MMaus]

I have now created an adapted version of this PR against master (just the imports in the test moved from javax.mail to jakarta.mail).