fix: self-signed leaf certificate is not self-issued

[JeyJeyGao]

Fix:

• added self-issued check for self-signed leaf certificate
• improved test coverage for x509 package
• fixed test case for revocation package

Resolves #126

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.70%. Comparing base (1683ddb) to head (8ab3059). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #236 +/- ## ========================================== + Coverage 91.29% 91.70% +0.40% ========================================== Files 30 30 Lines 2608 2616 +8 ========================================== + Hits 2381 2399 +18 + Misses 173 165 -8 + Partials 54 52 -2 ```

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