Fix Reported issue involving reoder and last

This should Resolve activerecord-hackery#418

Since the 4.1 version of `reverse_order!` is used, an additional check has been
 added to actually reorder the `ORDER BY` clause if there are non-nil elements
 of it.

Additionally, a spec has been added to ensure proper ordering takes place.

lib/squeel/adapters/active_record/4.1/relation_extensions.rb

@@ -225,7 +225,9 @@ def build_from
         def build_order(arel)
           orders = order_visit(dehashified_order_values)
           orders = orders.uniq.reject(&:blank?)
-          orders = reverse_sql_order(orders) if reverse_order_value && !reordering_value
+          if reverse_order_value && (!reordering_value || Array(order_values).any?(&:present?))
+            orders = reverse_sql_order(orders)
+          end
 
           arel.order(*orders) unless orders.empty?
         end

spec/squeel/adapters/active_record/relation_extensions_spec.rb

@@ -930,6 +930,11 @@ module ActiveRecord
             sql.should_not match /ORDER BY/
           end
 
+          it 'reverses the order if used with last' do
+            expected = Person.all.sort_by { |p| p.id }.last
+            result = Person.reorder(:id).last
+            result.should eq(expected)
+          end
         end
 
         describe '#from' do