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
```