fix: handle BigInts that has a .toJSON property

Checklist

[x] I have ensured my pull request is not behind the main or master branch of the original repository.
[x] I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
[x] I have written a commit message that passes commitlint linting.
[x] I have ensured that my code changes pass linting tests.
[x] I have ensured that my code changes pass unit tests.
[x] I have described my pull request and the reasons for code changes along with context if necessary.

Problem

The recent changes introduced in https://github.com/ladjs/superagent/pull/1773 restricts the usage of BigInt with the .send() method, even if the BigInt object has a .toJSON method. In scenarios where developers want to serialize BigInt properties to JSON, this becomes a limitation.

Change

This pull request modifies the existing checks on BigInt values passed to the .send() method. With this change, if a BigInt property has a .toJSON method, it will not throw the "Cannot serialize BigInt value to json" error.

ladjs / superagent