[backwards compatibility] [mustella] horizontalGap for CheckBox defaults to 5 instead of 2

Reproduction:
1. Compile & run the following w/ Flex 2.0.1 and the latest Flex 3 build
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:Canvas id="myCanvas" height="150" width="300">
<mx:VBox>
<mx:CheckBox id="myCheckBox"/>
<mx:Label text="

"/>
</mx:VBox>
</mx:Canvas>
</mx:Application>

Actual Results:
In Flex 2.0.1 the label shows that the default horizontalGap is 2. In Flex 3 the label shows it's 5.

Expected Results:
According to the documentation, horizontalGap should default to 2. (Button has the same style and it still defaults to 2 even in Flex 3)

Workaround:
None