[Webtest] StoreInputFieldAttribute for a Checkbox

[Soula, William]

I was messing around with the storeInputFieldAttribute function and noticed 
that for attribute="checked" the values returned are "checked" and "false".  
Whereas, the setCheckbox step only accepts "true" and "false" for checked.  
Shouldn't these two steps line up such that you can store the checked attribute 
of a checkbox and then right away be able to set a checkbox with it?  In order 
to do this I've had to add a scriptStep to change "checked" to "true"