use bang bang for #destroyed?

Fixes #57

lib/paranoia.rb

@@ -56,7 +56,7 @@ module Paranoia
   end
   end
 
 
   def destroyed?
   def destroyed?
-    !self.deleted_at.nil?
+    !!deleted_at
   end
   end
   alias :deleted? :destroyed?
   alias :deleted? :destroyed?
 end
 end