use arel nodes to represent non-string `order_values`.

This fixes a bug when merging relations of different classes.

```
Given:
  Post.joins(:author).merge(Author.order(name: :desc)).to_sql

Before:
 SELECT "posts".* FROM "posts"
   INNER JOIN "authors" ON "authors"."id" = "posts"."author_id"
   ORDER BY "posts"."name" DESC

After:
 SELECT "posts".* FROM "posts"
   INNER JOIN "authors" ON "authors"."id" = "posts"."author_id"
   ORDER BY "authors"."name" DESC
```

1 files changed, 1 insertions, 5 deletions

diff --git a/activerecord/test/cases/relation/mutation_test.rb b/activerecord/test/cases/relation/mutation_test.rb
index 020fb24afa..a70f979442 100644
--- a/activerecord/test/cases/relation/mutation_test.rb
+++ b/activerecord/test/cases/relation/mutation_test.rb
@@ -7,10 +7,6 @@ module ActiveRecord
       extend ActiveRecord::Delegation::DelegateCache
       inherited self
 
-      def arel_table
-        Post.arel_table
-      end
-
       def connection
         Post.connection
       end
@@ -21,7 +17,7 @@ module ActiveRecord
     end
 
     def relation
-      @relation ||= Relation.new FakeKlass.new('posts'), :b
+      @relation ||= Relation.new FakeKlass.new('posts'), Post.arel_table
     end
 
     (Relation::MULTI_VALUE_METHODS - [:references, :extending, :order]).each do |method|