http2/compat: make req.trailers have a null prototype

In HTTP/2 compatibility mode, make request trailers have a null
prototype, matching the behavior of regular headers and trailers.

Author: mcollina

lib/internal/http2/compat.js

@@ -319,7 +319,7 @@ class Http2ServerRequest extends Readable {
     // initialization using Object.create(null) in HTTP/2 is intentional.
     this[kHeaders] = headers;
     this[kRawHeaders] = rawHeaders;
-    this[kTrailers] = {};
+    this[kTrailers] = { __proto__: null };
     this[kRawTrailers] = [];
     this[kStream] = stream;
     this[kAborted] = false;

test/parallel/test-http2-compat-serverrequest-trailers.js

@@ -22,6 +22,7 @@ server.listen(0, common.mustCall(function() {
     request.on('data', common.mustCallAtLeast((chunk) => data += chunk));
     request.on('end', common.mustCall(() => {
       const trailers = request.trailers;
+      assert.strictEqual(Object.getPrototypeOf(trailers), null);
       for (const [name, value] of Object.entries(expectedTrailers)) {
         assert.strictEqual(trailers[name], value);
       }