[krzysdz]

This PR is based on #1560, with the following changes:

• req._unset is initialised to {} in both Node.js and browser versions (should prevent this._unset is undefined in the browser #1573),
• there are no changes related to Content-Type, because it is possible to unset it¹.

Warning

The new tests run only on Node.js, because only the Node.js code sets the default Accept-Encoding. The tests do not prevent a regression like #1573, because .unset() tests can be found only in test/node/.

Closes #1860

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
• I have ensured that my code changes pass linting tests.
  None of the lint warnings are from my code.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.

Footnotes

1. Except that request.post('/sth').send({ name: 'tj' }).unset('Content-Type').end() will fail, because the data cannot be serialised as serialisation function is chosen in ._end() based on the Content-Type. This probably should be another issue. ↩