doc, test: add note to response.getHeaders

* also correct language for the same note for querystring.parse
* add assertions for said note

PR-URL: https://github.com/nodejs/node/pull/12887
Fixes: https://github.com/nodejs/node/issues/12885
Refs: https://github.com/nodejs/node/pull/12883
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/http.md

@@ -556,7 +556,7 @@ Returns `request`.
 added: v0.1.17
 -->
 
-This class inherits from [`net.Server`][] and has the following additional events:
+This class inherits from [`net.Server`][] and has the following additional events:
 
 ### Event: 'checkContinue'
 <!-- YAML
@@ -982,6 +982,11 @@ header-related http module methods. The keys of the returned object are the
 header names and the values are the respective header values. All header names
 are lowercase.
 
+*Note*: The object returned by the `response.getHeaders()` method _does not_
+prototypically inherit from the JavaScript `Object`. This means that typical
+`Object` methods such as `obj.toString()`, `obj.hasOwnProperty()`, and others
+are not defined and *will not work*.
+
 Example:
 
 ```js

doc/api/querystring.md

@@ -68,9 +68,9 @@ For example, the query string `'foo=bar&abc=xyz&abc=123'` is parsed into:
 ```
 
 *Note*: The object returned by the `querystring.parse()` method _does not_
-prototypically extend from the JavaScript `Object`. This means that the
+prototypically inherit from the JavaScript `Object`. This means that typical
-typical `Object` methods such as `obj.toString()`, `obj.hasOwnProperty()`,
+`Object` methods such as `obj.toString()`, `obj.hasOwnProperty()`, and others
-and others are not defined and *will not work*.
+are not defined and *will not work*.
 
 By default, percent-encoded characters within the query string will be assumed
 to use UTF-8 encoding. If an alternative character encoding is used, then an

test/parallel/test-http-mutable-headers.js

@@ -43,8 +43,9 @@ const s = http.createServer(common.mustCall((req, res) => {
   switch (test) {
     case 'headers':
       // Check that header-related functions work before setting any headers
-      // eslint-disable-next-line no-restricted-properties
+      const headers = res.getHeaders();
-      assert.deepEqual(res.getHeaders(), {});
+      const exoticObj = Object.create(null);
+      assert.deepStrictEqual(headers, exoticObj);
       assert.deepStrictEqual(res.getHeaderNames(), []);
       assert.deepStrictEqual(res.hasHeader('Connection'), false);
       assert.deepStrictEqual(res.getHeader('Connection'), undefined);
@@ -72,6 +73,7 @@ const s = http.createServer(common.mustCall((req, res) => {
       assert.strictEqual(res.getHeader('x-test-header2'), 'testing');
 
       const headersCopy = res.getHeaders();
+      assert.strictEqual(Object.getPrototypeOf(headersCopy), null);
       // eslint-disable-next-line no-restricted-properties
       assert.deepEqual(headersCopy, {
         'x-test-header': 'testing',