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);