Support reversing of Arel expressions in reverse_order

The current behaviour of reverse_order, when presented with an
existing order that is an Arel expression, is to ignore it.
This isn't quite in keeping with how it handles Arel attributes
(which it reverse) or arbitrary strings (which throw an exception).

The easy path would be to throw an exception, but we don't have to.
Any SQL expression that is a valid default sort expression can be
reversed with DESC.

It follows that if there's an existing Arel::Nodes::NodeExpression
that is not already wrapped with an Arel::Nodes::Ordering, then
it can be reversed with DESC.

activerecord/lib/active_record/relation/query_methods.rb

@@ -1360,6 +1360,8 @@ def reverse_sql_order(order_query)
             o.desc
           when Arel::Nodes::Ordering
             o.reverse
+          when Arel::Nodes::NodeExpression
+            o.desc
           when String
             if does_not_support_reverse?(o)
               raise IrreversibleOrderError, "Order #{o.inspect} cannot be reversed automatically"

activerecord/test/cases/relations_test.rb

@@ -306,6 +306,11 @@ def test_reverse_order_with_function
     assert_equal topics(:third).title, topics.first.title
   end
 
+  def test_reverse_arel_order_with_function
+    topics = Topic.order(Topic.arel_table[:title].lower).reverse_order
+    assert_equal topics(:third).title, topics.first.title
+  end
+
   def test_reverse_arel_assoc_order_with_function
     topics = Topic.order(Arel.sql("lower(title)") => :asc).reverse_order
     assert_equal topics(:third).title, topics.first.title