fix(ext/fetch): ignore user content-length header (#15555)

Previously if a user specified a content-length header for an POST
request without a body, the request would contain two `content-length`
headers. One added by us, and one added by the user.

This commit ignores all content-length headers coming from the user,
because we need to have the sole authority on the content-length because
we transmit the body.

cli/tests/unit/fetch_test.ts

@@ -760,6 +760,68 @@ Deno.test(
   },
 );
 
+Deno.test(
+  {
+    permissions: { net: true },
+  },
+  async function fetchUserSetContentLength() {
+    const addr = "127.0.0.1:4501";
+    const bufPromise = bufferServer(addr);
+    const response = await fetch(`http://${addr}/blah`, {
+      method: "POST",
+      headers: [
+        ["Content-Length", "10"],
+      ],
+    });
+    await response.arrayBuffer();
+    assertEquals(response.status, 404);
+    assertEquals(response.headers.get("Content-Length"), "2");
+
+    const actual = new TextDecoder().decode((await bufPromise).bytes());
+    const expected = [
+      "POST /blah HTTP/1.1\r\n",
+      "content-length: 0\r\n",
+      "accept: */*\r\n",
+      "accept-language: *\r\n",
+      `user-agent: Deno/${Deno.version.deno}\r\n`,
+      "accept-encoding: gzip, br\r\n",
+      `host: ${addr}\r\n\r\n`,
+    ].join("");
+    assertEquals(actual, expected);
+  },
+);
+
+Deno.test(
+  {
+    permissions: { net: true },
+  },
+  async function fetchUserSetTransferEncoding() {
+    const addr = "127.0.0.1:4501";
+    const bufPromise = bufferServer(addr);
+    const response = await fetch(`http://${addr}/blah`, {
+      method: "POST",
+      headers: [
+        ["Transfer-Encoding", "chunked"],
+      ],
+    });
+    await response.arrayBuffer();
+    assertEquals(response.status, 404);
+    assertEquals(response.headers.get("Content-Length"), "2");
+
+    const actual = new TextDecoder().decode((await bufPromise).bytes());
+    const expected = [
+      "POST /blah HTTP/1.1\r\n",
+      "content-length: 0\r\n",
+      `host: ${addr}\r\n`,
+      "accept: */*\r\n",
+      "accept-language: *\r\n",
+      `user-agent: Deno/${Deno.version.deno}\r\n`,
+      "accept-encoding: gzip, br\r\n\r\n",
+    ].join("");
+    assertEquals(actual, expected);
+  },
+);
+
 Deno.test(
   {
     permissions: { net: true },

ext/fetch/lib.rs

@@ -287,7 +287,7 @@ where
           .map_err(|err| type_error(err.to_string()))?;
         let v = HeaderValue::from_bytes(&value)
           .map_err(|err| type_error(err.to_string()))?;
-        if name != HOST {
+        if !matches!(name, HOST | CONTENT_LENGTH) {
           request = request.header(name, v);
         }
       }