make user-agent and content-type overwritable with undefined

[Uzlopak]

trafficstars

@budarin I open this PR to see if the unit tests run.

Checklist

• [ ] run npm run test and npm run benchmark
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [ ] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[budarin]

Thanks!

This covers the necessary requirements 🚀

[Uzlopak]

Well it is based on your work. Lets see if the others agree