javascript-enabled keyboard shortcuts cannot override browser shortcut (e.g ctrl-s)

[Chunlei Wu]

To re-produce this bug:

1. Goto: http://www.openjs.com/scripts/events/keyboard_shortcuts/

2. In "Live Evaluation" box, enter this code:

shortcut.add("Ctrl+s",function() {
 alert("Hi there!");
});

3. click "Execute".

4. Then press "Ctrl+s". It does not show the expected "alert" dialog, instead, browser's "save page as..." action is triggered.

Basically, if the shortcut assigned by javascript is used by browser globally, it won't work. Other un-used shortcuts work fine.

Tested above in Chrome/Firefox, "ctrl+s" works as expected (the browser action is overridden)