GUAC-888: Init socket last_write_timestamp during alloc, otherwise keep-alive math fails until data has been written, which may result in the keep-alive never being sent.

src/libguac/socket.c

@@ -152,6 +152,7 @@ guac_socket* guac_socket_alloc() {
     socket->__written = 0;
     socket->data = NULL;
     socket->state = GUAC_SOCKET_OPEN;
+    socket->last_write_timestamp = guac_timestamp_current();
 
     /* Init members */
     socket->__instructionbuf_unparsed_start = socket->__instructionbuf;