Fix: MSH offset on array getters and setters

[LucasMontorio]

Description

Fixes #123

The MSH-1 field has a special behaviour, and does not appear in the output of the HL7 string for this segment, hence breaking the indexing of the other fields by causing an offset.
This PR fixes the offset and restores the order of indexes as expected.

Checklist

• 💡 The PR has a concise and explicit title
• 📝 The PR contains a clear description of the changes
• 🧪 There are unit tests that prove that the fix is effective or that the feature works

[Stratus3D]

Rubocop is failing.

[LucasMontorio]

Rubocop is failing.

Thanks, it's fixed

[LucasMontorio]

Rubocop is failing.

Thanks, it's fixed

Hi @Stratus3D, is there anything else blocking in the PR according to you? :)

[Stratus3D]

Not sure I understate the index offset logic. I don't have time to put into this project, so I'll leave this PR up to you. I'd be nice if one of the other old maintainers could weight in on PRs like this.

[LucasMontorio]

Not sure I understate the index offset logic. I don't have time to put into this project, so I'll leave this PR up to you. I'd be nice if one of the other old maintainers could weight in on PRs like this.

I'll keep it open at least until the preparation of the next version bump, in hope for another review. Thanks for checking it anyway