fix: unify body and query serialisation behaviour

[MaoShizhong]

Because

#1591 changed things to use qs to serialise form data. However, that change set qs.stringify as the body serialisation function i.e. default options. The existing query serialisation uses qs.stringify with indices: false and strictNullHandling: true options (src/node/index.js line 679).

Existing documentation indicates that body serialisation with duplicated object keys should result in the same behaviour as the existing query serialisation. However, qs.stringify with default options results in a different outcome, as described in the issue this PR closes.

This PR

• Changes body serialisation function to match existing query serialisation function and options, bringing its behaviour in line with existing documentation
• Adds test for serialising duplicated keys

Linked issue

Closes #1798

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.
• 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.

[MaoShizhong]

@titanism anything extra I might need to fix the above?

[titanism]

v10.1.0 released to npm, thank you @MaoShizhong

ref: https://github.com/ladjs/superagent/releases/tag/v10.1.0

p.s. our efforts to maintain this repo are supported by @forwardemail