Fix: Don't automatically manage headers in doRequest

[vaxerski]

This cumbers the availability of the API and leads to bad requests that should not be bad.

I am building a reverse-proxy with pistache, and the client API cumbers my ability to do so properly:

• The client API always drops the UA and replaces it with pistache's.
• The client API always adds the Host header and this leads to bad requests when we want to set the host header manually.

In this MR, I drop both. Instead, pistache's UA and Host are added iff they are not added by the client.

[kiplingw]

@vaxerski, please don't forget to bump the patch version per our documentation since you've altered the underlying implementation.

[vaxerski]

uhh... where is that documentation? I've checked the website and the repo and can't find anything related to contributing guidelines.

[kiplingw]

See here.

[vaxerski]

ah, my bad, I got a zoomer brain. Thanks!

[vaxerski]

done, I've also reworded the commit, I think the linter should be happy now.