Undici didn't update `Content-Type` header for `multipart/form-data` requests after following redirects which leads to mismatch in request body and header

[AndriiHe]

Bug Description

There is a problem when sending a multipart/form-data request that follows redirects. After the redirect, undici overrides the request body but does not update the Content-Type header, which causes an error when parsing the body. The updated content type just ignored here.

Reproducible By

const { createServer } = require("http");
const { fetch, FormData } = require("undici");

createServer((req, res) => {
  if (req.method === "POST" && req.url === "/first") {
    res.writeHead(307, {
      Location: "http://localhost:3000/second",
    });
    res.end();
  } else if (req.method === "POST" && req.url === "/second") {
    console.log("Request headers:\n", req.headers);

    let body = "";
    req.on("data", (chunk) => {
      body += chunk;
    });

    req.on("end", () => {
      console.log("Request body:\n", body);
      res.writeHead(200);
      res.end("Request received");
    });
  }
}).listen(3000);

// Make a request to the first endpoint using undici
async function makeRequest() {
  const formData = new FormData();
  formData.append("test", "data");

  const response = await fetch("http://localhost:3000/first", {
    method: "POST",
    body: formData,
    redirect: "follow",
  });

  console.log("Response status:\n", response.status);
}

makeRequest().catch(console.error);

Expected Behavior

The Content-Type header should be updated along with the body after a redirect.

Logs & Screenshots

Request headers:
 {
  host: 'localhost:3000',
  connection: 'keep-alive',
  'content-type': 'multipart/form-data; boundary=----formdata-undici-073413599880',
  accept: '*/*',
  'accept-language': '*',
  'sec-fetch-mode': 'cors',
  'user-agent': 'undici',
  'accept-encoding': 'gzip, deflate',
  'content-length': '127'
}
Request body:
 ------formdata-undici-045095077684
Content-Disposition: form-data; name="test"

data
------formdata-undici-045095077684--

Environment

Additional context