Can't remove default headers. Can't overwrite `"sec-fetch-mode"`, `"connection"` headers.

• I can't remove any default header.
• Setting "connection": "keep-alive" throws an error.
• I can't overwrite default "sec-fetch-mode" header correctly.

1. I can't remove the default headers

I can't remove the default headers: host, connection, accept, accept-encoding, accept-language, sec-fetch-mode, user-agent.

For example, curl allows to remove any header (even host header):
curl http://127.0.0.1:3000 -H 'User-Agent:' -H 'Accept:' -H 'Host:'
And yes, it works.

Using of "" will send "" as the header's value, using of null will send "null" as the header's value.

2. "connection": "keep-alive"

Also, if I manually set connection: "keep-alive" I get the error (code: 'UND_ERR_INVALID_ARG').

const resp = await fetch(url, {
    headers: {
        "connection": "keep-alive"
    }
});

It's important for code compatibility reason — since node-fetch uses connection: close by default.

3. "sec-fetch-mode"

It uses "sec-fetch-mode": "cors" by default.

As I said above, I can't remove this header (as well as any other default header).

More over, I can't change it correctly.

For example, using of:

await fetch(url, {
    headers: {
        "sec-fetch-mode": "navigate"
    }
});

Will produce a request with sec-fetch-mode: navigate, cors, while it should be sec-fetch-mode: navigate.

"undici": "4.16.0"

Start by reproducing the three header cases against Undici 4.16.0 and tracing the fetch request path that builds and validates default headers. Done means default headers can be removed or replaced as described, setting connection to keep-alive no longer raises UND_ERR_INVALID_ARG, and sec-fetch-mode is sent only with the requested value.