Checkboxradio: change fires twice when hitting space on the widget

[gabrielschulhof]

Steps to reproduce:

1. Visit http://jsbin.com/IFolanOW/422/
2. Press TAB until the checkbox is highlighted
3. Hit space

At this point you will find that change fires twice, recording the value of the input's checked property in the "console".

For comparison, click on the checkbox instead. The problem is that the space bar triggers a vclick - yes, a vclick, which causes the checkboxradio code to trigger a change event. However, the input also natively triggers change in response to the spacebar. That makes two change events for each space bar press.