in_message->string() not idempotent

I was surprised to discover that in_message->string() can only be invoked once, or else it returns a value that is not actually a string.

This works:

ws_endpoint.on_message = [](shared_ptr<WsServer::Connection> connection, shared_ptr<WsServer::InMessage> in_message) {

  const std::string s = in_message->string();

  cout << "Server: Message received: \"" << s << "\" from " << connection.get() << endl;

This does not:

ws_endpoint.on_message = [](shared_ptr<WsServer::Connection> connection, shared_ptr<WsServer::InMessage> in_message) {

  cout << "Server: Message received: \"" << in_message->string() << "\" from " << connection.get() << endl;

  const std::string s = in_message->string();

I discovered it because I immediately send the string to another library for parsing.

Seems like in_message->string() should either be idempotent (repeatable) or throw an Exception on repeated calls. At the very least, perhaps the examples could be updated to illustrate this behavior.