Fix bug in noDuplicate invariant
Fix bug in noDuplicate invariant

ws used by noDuplicates is actually a list of list of elements which 
will pretty rarely raise any flags even if the StackSet actually does
contain duplicates. This patch concatenates ws to ensure the quickcheck
property tests accurately.

ohlus/xmonad,ericssmith/xmonad,xmonad/xmonad,pjones/xmonad-test