fix(fetch-adapter): set correct Content-Type for Node FormData

[emiedonmokumo]

Summary

This pull request fixes an issue in the fetch adapter for Node.js where FormData requests were being sent with application/x-www-form-urlencoded instead of multipart/form-data; boundary=....

Problem

In Node.js, the fetch adapter did not correctly set the Content-Type header when sending FormData.
This caused requests using FormData to fail or be misinterpreted by servers expecting multipart/form-data with the proper boundary.
The issue was confirmed in axios#6993 and affects only the Node.js environment; browser behavior remains correct.

Solution

• Detects if the request body is FormData and has a getHeaders() method (from the form-data package).
• Uses data.getHeaders()['content-type'] to get the correct Content-Type header including the boundary.
• Sets the header in the request before sending.
• Added a proper Node.js test using the form-data package to verify the Content-Type header is correctly set with a boundary:

  const FormData = (await import('form-data')).default;
  const form = new FormData();
  form.append('foo', 'bar');
  
  server = await startHTTPServer((req, res) => {
    const contentType = req.headers['content-type'];
    expect(contentType).to.match(/^multipart\/form-data; boundary=/i);
    res.end('OK');
  });
  
  await fetchAxios.post('/form', form);

[jasonsaayman]

looks good i just question if need to include chai? i think its unnecessary to include it as we use the built in assertions

[emiedonmokumo]

Alright, thanks for the response, will definitely make the update

[Copilot]

Pull Request Overview

Fixes Content-Type header handling for FormData in Node.js fetch adapter to properly set multipart/form-data with boundary instead of the incorrect application/x-www-form-urlencoded.

• Replaces complex header parsing logic with direct usage of FormData's getHeaders() method
• Adds Node.js-specific test to verify correct multipart/form-data Content-Type with boundary
• Simplifies browser vs Node.js environment handling

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
lib/helpers/resolveConfig.js	Updates FormData Content-Type handling to use getHeaders() for Node.js FormData
test/unit/adapters/fetch.js	Adds test case to verify correct Content-Type header for Node.js FormData

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[jasonsaayman]

LGTM, thanks for the contribution 🔥

[emiedonmokumo]

Looking forward to making more contributions. Any tips on how I can easily understand the folder structure?

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[jasonsaayman]

Looking forward to making more contributions. Any tips on how I can easily understand the folder structure?

honestly i only got used to this after a real long time, this is something i hope to vastly improve in the next version as the on-ramp is quite steep and it should not be. but i would say taking up more issue and trying to solve them can help with this

[emiedonmokumo]

Thank you very much.

[github-actions]

Hi, @emiedonmokumo! This PR has been published in v1.12.0 release. Thank you for your contribution ❤️!