src: fix query/fragment serialization in URL::SerializeURL

These are presumably typos.

PR-URL: nodejs#41759
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>

src/node_url.cc

@@ -1576,10 +1576,10 @@ std::string URL::SerializeURL(const struct url_data* url,
     }
   }
   if (url->flags & URL_FLAGS_HAS_QUERY) {
-    output = "?" + url->query;
+    output += "?" + url->query;
   }
   if (!exclude && url->flags & URL_FLAGS_HAS_FRAGMENT) {
-    output = "#" + url->fragment;
+    output += "#" + url->fragment;
   }
   return output;
 }