Remove default fetch headers added by undici, closes #139

Specifically, remove `Accept`, `Accept-Language`, `Sec-Fetch-Mode`
and `User-Agent`, unless they are explicitly added by the user.
`User-Agent` is particularly important to remove as many APIs (e.g.
GitHub), require it to be set, meaning code was working locally but
not when deployed.

The implementation of this relies on some internal `undici` exports.
We were already using some internal symbols for accessing hidden
`Request`/`Response` state, so this isn't too bad. The version of
`undici` is now pinned to an exact version though, so if internal
changes are made, they shouldn't affect end-users. We'll have to
explicitly upgrade and run our tests again.

Author: mrbbot

package-lock.json

@@ -38,7 +38,7 @@
     "@miniflare/core": "2.0.0-rc.5",
     "@miniflare/shared": "2.0.0-rc.5",
     "http-cache-semantics": "^4.1.0",
-    "undici": "^4.11.1"
+    "undici": "4.12.1"
   },
   "devDependencies": {
     "@miniflare/shared-test": "2.0.0-rc.5",

packages/cache/package.json

@@ -44,7 +44,7 @@
     "dotenv": "^10.0.0",
     "kleur": "^4.1.4",
     "set-cookie-parser": "^2.4.8",
-    "undici": "^4.11.1"
+    "undici": "4.12.1"
   },
   "devDependencies": {
     "@miniflare/durable-objects": "2.0.0-rc.5",