Fix for Key.enabled issue #1190

For https://github.com/photonstorm/phaser/issues/1190 

I was just going to update the documentation, but after looking at the code it appeared that the "correct" work-about is actually the following, which seems like enough of a magical incantation to handled.

```javascript
key.reset(false);
key.enabled = false;
````

If `reset` is called without any parameters it will be "hard" and eliminate any callbacks; and the key will always be re-enabled by `reset`.