fix(node): preserve http number header type with `setHeader`

[Hebilicious]

🔗 Linked issue

fix https://github.com/unjs/unenv/issues/118

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• [x] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [ ] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR changes the odd unenv behaviour that joined multiple value headers instead of preserving the array. This brings setHeader behaviour in line with appendHeader when it comes to multiple values headers.

📝 Checklist

• [x] I have linked an issue or discussion.
• [ ] I have updated the documentation accordingly.