Better support for hidden selects when applying Chosen

For jQuery, we first test the css('width') of a select. This always
returns a value for visible selects, but it will return false
for hidden selects with no style set (inline or class based).

For Prototype, we first check the getStyle('width') of a select. This
behaves much the same way as jQuery's css('width'), fortunately.
When a field is hidden, it doesn't have a width and the search
fields width gets set to "-10px". So let's make sure there's a value!