Add missing kOutHeaders property

[tomaratyn]

The absence of this property caused Node's OutgoingMessage.prototype.setHeader function to fail as it checks for null but not for falsey or undefined.

When setHeader fails with unenv then nuxt can have problems properly rendering error pages for API routes/middleware.

🔗 Linked issue

❓ 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

The current re-implementation of ServerResponse is missing the property kOutHeaders. The absence of this property means that it's undefined but Node tests using === null to check if the value needs to be initialized (see https://github.com/nodejs/node/blob/main/lib/_http_outgoing.js#L709).

Due to this when a Nuxt server route has an error it can fail to render since Nuxt will use Nitro's localFetch which in turn uses unenv's createFetch which in turn uses unenv's ServerResponse.

📝 Checklist

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

[tomaratyn]

Actually that property can't be a string, it has to be a Symbol that is not exported by Node. This approach won't work.