test: fix test case in test-http2-respond-file-304.js

PR-URL: #26565
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

Author: gengjiawen

test/parallel/test-http2-respond-file-304.js

@@ -33,7 +33,7 @@ server.listen(0, () => {
 
   req.on('response', common.mustCall((headers) => {
     assert.strictEqual(headers[HTTP2_HEADER_STATUS], 304);
-    assert.strictEqual(headers[HTTP2_HEADER_CONTENT_TYPE, undefined]);
+    assert.strictEqual(headers[HTTP2_HEADER_CONTENT_TYPE], undefined);
   }));
 
   req.on('data', common.mustNotCall());