Fix bug when the previous data sent was larger

[mellado]

There is a bug when you send some data, and then in a second communication with smaller data you can see at the end of the the previous last part of the larger data. Then the size obtained is the same in both cases.

[guitronimo]

This same problem still exists when using option 1 or 3 from Server to client.

This seem to have fixed it:

client_example:

// observer callback. will be called for every new message received by the server
void onIncomingMsg(const char * msg, size_t size) {
    std::cout << "Got msg from server: " << std::string(msg, size) << "\n";
}```