fix(ext/fetch): Properly cancel upload stream when aborting

[nayeemrmn]

Discovered bug while exploring denoland/std#1214.

const abortController = new AbortController();
await fetch(
  "https://deno.land/",
  {
    method: "POST",
    body: new ReadableStream({
      pull(controller) {
        abortController.abort();
        controller.enqueue(new Uint8Array([1, 2, 3, 4]));
      },
    }),
    signal: abortController.signal,
  },
);

main:

error: Uncaught (in promise) TypeError: Cannot cancel a locked ReadableStream.
        abortController.abort();
                        ^
    at ReadableStream.cancel (deno:ext/web/06_streams.js:3199:11)
    at InnerBody.cancel (deno:ext/fetch/22_body.js:126:29)
    at abortFetch (deno:ext/fetch/26_fetch.js:502:45)
    at ...

This PR:

error: Uncaught AbortError: Ongoing fetch was aborted.
        abortController.abort();
                        ^
    at abortFetch (deno:ext/fetch/26_fetch.js:512:19)
    at ...

[bartlomieju]

LGTM, confirmed that test fails on main. @lucacasonato any objections?