getHeaders should add content length if known

[wmertens]

If all the form data has known lengths, it would be nice if getHeaders() automatically included the Content-Length header.

[XenoBino]

Until it's supported, use:

const formData = new FormData();
// ...
const buffer = formData.getBuffer();

// or axios, or whatever
fetch(url, {
  headers: {
    ..formData.getHeaders(),
    "Content-Length": buffer.byteLength.toString()
  },
  body: buffer
})