fix(tools): FixtureHeader join (for rlp_modifier)

[marioevz]

🗒️ Description

Small fix to the join method used when a block has an rlp_modifier.

I used this to debug a Besu issue, and I was setting the rlp_modifier = Header(state_root='0x...'), and set the state root to a string, hoping that it would be automatically converted to the appropriate type, but that didn't happen.

With the change in the PR, the state root, and all other fields, are automatically converted to the correct type.

🔗 Related Issues

None

✅ Checklist

• All: Set appropriate labels for the changes.
• All: Considered squashing commits to improve commit history.
• All: Added an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• Tests: Included the type and version of evm t8n tool used to locally execute test cases: e.g., ref with commit hash or geth 1.13.1-stable-3f40e65.
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.

[danceratopz]

Added tests - it works! 🙂