Allow building without rtti (#487)

Since factory returns a ProxyConnectionState, setOnConnectionCallback
will be a ProxyConnectionState. The code already makes that assumption,
since it does not check of state return value. Using a
static_pointer_cast will allow the library to be build with rtti.

ixwebsocket/IXWebSocketProxyServer.cpp

@@ -57,7 +57,7 @@ namespace ix
         server.setOnConnectionCallback(
             [remoteUrl, remoteUrlsMapping](std::weak_ptr<ix::WebSocket> webSocket,
                                            std::shared_ptr<ConnectionState> connectionState) {
-                auto state = std::dynamic_pointer_cast<ProxyConnectionState>(connectionState);
+                auto state = std::static_pointer_cast<ProxyConnectionState>(connectionState);
                 auto remoteIp = connectionState->getRemoteIp();
 
                 // Server connection