feat: allow form data through default body serializer

[thatsprettyfaroutman]

I was trying to use openapi-fetch for file uploading, but by default that wasn't possible (using multipart/form-data). Looking at the source code I noticed that part of the solution for fixing it seemed to be missing as defaultBodySerializer never allows FormData to be passed through it:

if (requestInit.body) {
  // Here, bodySerializer always returns a JSON string by default
  requestInit.body = bodySerializer(requestInit.body);
  // remove `Content-Type` if serialized body is FormData; browser will correctly set Content-Type & boundary expression
  if (requestInit.body instanceof FormData) {
    requestInit.headers.delete("Content-Type");
  }
}

I'm trying to fix that in this PR by allowing FormData to be passed through the defaultBodySerializer enabling file uploads with multipart/form-data by default.

Changes

Allow FormData to be passed through defaultBodySerializer as body to enable file uploading by default.

How to Review

Upload a file as part of multipart/form-data

Checklist

• [ ] Unit tests updated
• [ ] docs/ updated (if necessary)
• [ ] pnpm run update:examples run (only applicable for openapi-typescript)