test: increase test coverage for http.OutgoingMessage.appendHeader()

Signed-off-by: Juan José Arboleda <soyjuanarbol@gmail.com>
PR-URL: #55467
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>

Author: juanarbol

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

@@ -65,6 +65,10 @@ const server = createServer(
       res.write('BODY');
       res.end();
 
+      assert.throws(() => res.appendHeader(), {
+        code: 'ERR_HTTP_HEADERS_SENT',
+      });
+
       assert.deepStrictEqual(res.getHeader('X-Res-a'), ['AAA', 'BBB', 'CCC']);
       assert.deepStrictEqual(res.getHeader('x-res-a'), ['AAA', 'BBB', 'CCC']);
       assert.deepStrictEqual(