http2: use Object.create(null) for getHeaders

refactor `getHeaders` to initialize headers using
`Object.create(null)`

Refs: #29829

PR-URL: #33188
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sebastiaan Deckers <sebdeckers83@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/internal/http2/compat.js

@@ -555,7 +555,8 @@ class Http2ServerResponse extends Stream {
   }
 
   getHeaders() {
-    return { ...this[kHeaders] };
+    const headers = ObjectCreate(null);
+    return ObjectAssign(headers, this[kHeaders]);
   }
 
   hasHeader(name) {

test/parallel/test-http2-compat-serverresponse-headers.js

@@ -102,11 +102,13 @@ server.listen(0, common.mustCall(function() {
     response.setHeader(real, expectedValue);
     const expectedHeaderNames = [real];
     assert.deepStrictEqual(response.getHeaderNames(), expectedHeaderNames);
-    const expectedHeaders = { [real]: expectedValue };
+    const expectedHeaders = Object.create(null);
+    expectedHeaders[real] = expectedValue;
     assert.deepStrictEqual(response.getHeaders(), expectedHeaders);
 
     response.getHeaders()[fake] = fake;
     assert.strictEqual(response.hasHeader(fake), false);
+    assert.strictEqual(Object.getPrototypeOf(response.getHeaders()), null);
 
     assert.strictEqual(response.sendDate, true);
     response.sendDate = false;