1785: Fix bug #1660 (don't replace nested headers) r=mergify[bot] a=nextgens
## What type of PR?
bug-fix
## What does this PR do?
Don't replace nested headers (typically in forwarded/attached emails). This will ensure we don't break cryptographic signatures.
### Related issue(s)
- close#1660
## Prerequistes
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.
Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>
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