fix: handle BigInts that has a .toJSON property

[bjornua]

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.

Problem

The recent changes introduced in #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.

[titanism]

Fixed and released in v8.1.2 - thank you @bjornua

https://github.com/ladjs/superagent/releases/tag/v8.1.2