avoid restart user conversation on call the start method

[andersonba]

I found this bug when I configured the bot for multiple clients (multi-session).
In my configuration, the start method is called every time when the bot receives a message:

bot.session(sid, { store: storeFromDB(sid) })
  .hear(msg)
  .start();

The bot replies the hear method normally, but the start method dispatches the restart command, making the bot to lost the previous currentIdx.

Found

// store { currentIdx: 2 }
bot.hear(message).start();
// store { currentIdx: 0 }

Expected

// store { currentIdx: 2 }
bot.hear(message).start();
// store { currentIdx: 2 }