Sortable: check css with/height settings. Fixed #4345 - sortablle: CSS d...

The issue fixes the "4345 - Sortable: CSS defined widths / heights are ignored on helper" (http://bugs.jqueryui.com/ticket/4345#no2) from the bug tracker.

I've fixed it by checking if the item already has a size (width or height) set using a css. This is done by cloning the original element and checking if the size is the same.

I've added also a unit test to prove this fix in the "tests/unit/sortable/sortable_options.js" file.