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

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).

Related:

- https://github.com/socketio/socket.io/commit/ca9e994815aa2e31e0342e37ccdc2e9e8c5fd13c
- https://github.com/socketio/socket.io/issues/5215

[skip ci]