IMAPNestedMessage encoding sometimes different

*IMAPNestedMessage* should store the encoding because it can be different from the enclosing message.

Please see this commit for the required changes:

https://github.com/M66B/FairEmail/commit/61d078f92949da55dfe046848362e0bb80459c20

This happens with at least Gmail when sending an EML file as an attachments (which is rather common since Microsoft Outlook has a function to forward messages like this).

Start by reviewing the IMAPNestedMessage handling and the enclosing message's encoding behavior, then inspect commit 61d078f92949da55dfe046848362e0bb80459c20 for the required changes. Confirm the implementation preserves the nested message's own encoding when it differs from the enclosing message, including the Gmail EML-attachment case.

Written by the indexing model from the issue text.