Able to use `robot.send room: "some/roomUri", ...` as other adapters

This PR is first a new feature, but includes also a fix:
- [x] Fixes an error where the bot will stop sending messages and respond to them as soon as he'd send an empty message
  
    This might be fixing #3 as I couldn't reproduce but I am not sure at all

- [x] Using `robot.send {room: 'user/repo'}, 'hello'` is now working (for joined rooms of course).
    
    No need to know the room ID
    
    This works also with the URI to a channel