Fix IBufferWriterExtensions Write for unmanaged types

[pziezio]

Closes #798

Fixes extension method and adds a test to ensure a correct bahavior.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

[Sergio0694]

Thank you! 🙂