Prevent extra enter after submitting message

* Check _onChange so not set the state to the "\n".
* Seems that setting state from _onKeyDown triggers rendering, with
  state set to '', but then the _onChange event is called, somehow with
  the event.target.value set "\n".
* Is there some way to stop event propagation in the _onKeyDown?
* Tried event.stopPropgation in _onKeyDown, but that didn't work.