patch to support importing tables with composite primary keys

I'm using the composite primary keys plugin (http://compositekeys.rubyforge.org/) and found that I couldn't import tables with composite keys because ar-extensions didn't know how to check whether the primary key(s) were being inserted when the primary key was an array:

--- a/ar-extensions/lib/ar-extensions/import.rb
+++ b/ar-extensions/lib/ar-extensions/import.rb
@@ -185,7 +185,11 @@ class ActiveRecord::Base
       # Force the primary key col into the insert if it's not
       # on the list and we are using a sequence and stuff a nil
       # value for it into each row so the sequencer will fire later
-      if !column_names.include?(primary_key) && sequence_name && connection.prefetch_primary_key?
+      # add support for primary keys being an array rather than scalar
+      if ![primary_key].flatten.*.to_sym.to_set.subset?(column_names.*.to_sym.to_set) &&
+          sequence_name && connection.prefetch_primary_key?
          column_names << primary_key
          array_of_attributes.each { |a| a << nil }
       end