fix(sio-client): do not mangle the "_placeholder" attribute

The "_placeholder" attribute is used when sending binary data, and was
incorrectly mangled (converted to a random short property, like "it",
to reduce the bundle size).

This bug was introduced in [1], included in `socket.io-client@4.8.0`.

[1]: https://github.com/socketio/socket.io/commit/7085f0e3e46cd1fd41d952450b8d01b04de83daf

Related: https://github.com/socketio/socket.io/issues/5215