Closed Unassigned: get error when connect to websocket via proxy [347]

Hi,

I used casablanca(development branch lastest commit) to connect with websocket server via proxy.

```
websocket_callback_client client;
websocket_client_config client_config;
web::web_proxy wp(U("http://myproxyserver:8080"));
credentials cred("username","password");
wp.set_credentials(cred);
client_config.set_proxy(wp);
client = websocket_callback_client(client_config);

pplx::task<void> connectTask = client.connect(U(uri));
try {
connectTask.wait();
cout << "Connect success!" <<endl;

// Handle receive message

});

// Handle closing connection
client.set_close_handler([](websocket_close_status status, utility::string_t reason, std::error_code errCode) {
printf("Error exception:%s \n", errCode.message().c_str());
});

} catch (websocket_exception &e) {
}
```
unfortunately, after connect to server, immediately this connection was be closed, and throw this message "__Error exception:Undefined error: 0__"

All suggestions are welcome.
Thank you in advance.