fix hardcoded resource during uri construction

[antiqe]

I corrected the resource that was hardcoded during the creation of the URI.

The connection should be :

sio::client h;
h.connect("http://127.0.0.1:3000/socket.io");

Or maybe define the resource "/socket.io" by default if it is not specified during the connect call.

[melode11]

This can not be merged because it can not cover such case like:

sio::client h;
h.connect("http://127.0.0.1:3000");