[Rohan-Dedhia]

Description

Fixes #1860.

Currently, calling req.unset('Accept-Encoding') removes 'accept-encoding' from this.header, but src/node/index.js still unconditionally sets req.setHeader('Accept-Encoding', 'gzip, deflate') during request construction for non-HEAD requests.

This change:

1. Tracks unset header names in this._unset inside RequestBase.prototype.unset (cleared when .set() is called for that field).
2. Initializes _unset = {} in both client and node request constructors to prevent undefined errors.
3. Skips default 'gzip, deflate' assignment in src/node/index.js when accept-encoding has been unset.
4. Adds a regression test in test/node/basic.js asserting that req.unset('Accept-Encoding') does not send an Accept-Encoding header.

[titanism]

Thanks. #1861, #1865 and this PR took the same approach, and the combined commit credits #1861 as the first. Compared with them, it creates the unset record only when unset() is called, keeps the header unset after redirects, and removes headers regardless of capitalization. Closing in favor of that commit, released in v10.4.2 and now available on npm: https://github.com/forwardemail/superagent/releases/tag/v10.4.2

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