Fix #2317 force conversion to FormData for xhr.send

[Telroshan]

Description

In a multipart/form-data form, current implementation with the proxied FormData results in requests being malformed (wrong content type set to text/plain, FormData sent as [object FormData] in the payload)
Couldn't get the proxy object to work properly with XMLHTTPRequest's send method; even though calls are being forwarded to the underlying FormData through the proxy, the request won't set its type to multipart/form-data when appropriate and instead send a text/plain request.
One simple fix/workaround here is to copy the values into a new, actual FormData (not a proxied one)

Corresponding issue:
#2317
Also see this JSFiddle to reproduce the issue

Testing

Didn't know how to add a reliable test for this tbh, given that we mock the requests in our test suite

Checklist

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  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded