dispatchFetch does not accept node 18+ builtin FormData object

[yusefnapora]

Hey, I'm in the midst of updating a project to use miniflare 2.7 and found that passing a FormData object that's builtin to node 18 as the body of a dispatchFetch request no longer works. This was working prior to 2.7, so I'm assuming it's related to the new version of undici that got pulled in.

Importing FormData from undici instead of using the global one works, which is what I've done in the linked PR. But perhaps there's a way to get undici to recognize other FormData implementations? I haven't looked at undici's internals yet, so I don't know if that's possible.

We're also using a polyfill lib called @web-std/form-data that we maintain in-house, which also worked with miniflare 2.6 and below but no longer works with 2.7. We're thinking about updating it to use undici's FormData, which should solve the issue there, but if there's a simple way to get the existing implementation to play nice with miniflare that would be cool too.

[yusefnapora]

I should probably define "no longer works" 😄

If you give dispatchFetch the wrong kind of FormData, it seems to treat it as plain text, setting the content-type to text/plain;charset=utf-8 instead of multipart/form-data.

[mrbbot]

Hey! 👋 Looks like this is indeed an issue in undici. I've opened a PR to fix it. 👍