test: add test for http outgoing internal headers

PR-URL: https://github.com/nodejs/node/pull/13980
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

test/parallel/test-http-outgoing-internal-headers.js

@@ -0,0 +1,28 @@
+// Flags: --expose-internals
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+const { outHeadersKey } = require('internal/http');
+const { OutgoingMessage } = require('http');
+
+{
+  // tests for _headers get method
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage.getHeaders = common.mustCall();
+  outgoingMessage._headers;
+}
+
+{
+  // tests for _headers set method
+  const outgoingMessage = new OutgoingMessage();
+  outgoingMessage._headers = {
+    host: 'risingstack.com',
+    Origin: 'localhost'
+  };
+
+  assert.deepStrictEqual(outgoingMessage[outHeadersKey], {
+    host: ['host', 'risingstack.com'],
+    origin: ['Origin', 'localhost']
+  });
+}