Backport: Treat 'uuencode' as 'x-uuencode' for compatibility with some older ma…

[thorsteneckel]

…ilers

Closes #878. Fixes #844.

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Hi there!

We over at Zammad use this great gem for processing incoming mails. Thanks a lot for providing and maintaining it! Unfortunately we have huge compatibility issues with the latest 2.7 branch and we hadn't had the time to look deeper into it yet. That's why we're still relying on 2.6. However, we now have the issue that we can't process 'uuencode' / 'x-uuencode' Content-Transfer-Encoding mails. While investigating this issue we found out that it already has been fixed 4 years ago in master/2.7 but was never backported to 2.6. That's the purpose of this PR: Cherry-picking the fix from 2.7 into 2.6.

We would greatly appreciate a backport (and a new 2.6 release) so we can remove our fork and keep the original 2.6 version as a dependency while figuring things out with 2.7. 👨‍🔬