When decoding a multipart message from EML, use the original EML boundary instead of generating a new one

[termermc]

This is an implementation of my second suggested fix for #490.

A test for it is included, and all tests pass.

Let me know if this is a good solution for the #490. If it is, I'm hoping a 0.6.3 version can be released so I can update dependencies in some of my dependent projects to take advantage of this fix and the others available in HEAD.

Thank you!

[wneessen]

Thanks for contributing this PR @termermc. The change looks good to me and I think the solution is appropriate to address the issue you are facing in #490. I already tested it locally and it seems to work without issues. I'll approve it and let the CI run (macOS tests will likely fail - known issue).

As a side note, the 0.7.0 release is coming out in a few days. Although I'm a bit busy at the moment, there is only one PR to complete (tests have to be written) which should complete the release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (5254e5a) to head (0c11308).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #491      +/-   ##
==========================================
- Coverage   94.41%   93.45%   -0.96%     
==========================================
  Files          30       30              
  Lines        3618     3623       +5     
==========================================
- Hits         3416     3386      -30     
- Misses        148      188      +40     
+ Partials       54       49       -5     

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