Refactor: Handle missing Content-Type as plain text

[wneessen]

Added a new example email and modified tests to include an RFC 5322 format. Refactored the code to handle cases where the Content-Type header is missing by defaulting to plain text (7bit, US-ASCII). Deleted redundant test cases that no longer apply.

This PR closes #271

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.26%. Comparing base (3ca2968) to head (69b1dc2).

Files	Patch %	Lines
eml.go	71.42%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #272 +/- ## ========================================== - Coverage 85.31% 85.26% -0.06% ========================================== Files 25 25 Lines 2070 2076 +6 ========================================== + Hits 1766 1770 +4 - Misses 178 180 +2 Partials 126 126 ```

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