Fix generated projection fields in group by query

Closes #21922

Let `Book(id, author_id)`, `Photo(id, book_id, author_id)` and `Author(id)`

Running `Book.group(:author_id).joins(:photos).count` will produce:

* Rails 4.2 - conflicts `author_id` in both projection and group by:
```sql
SELECT COUNT(*) AS count_all, author_id AS author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY author_id
```

* Master (9d02a25) - conflicts `author_id` only in projection:
```sql
SELECT COUNT(*) AS count_all, author_id AS author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY "books"."author_id"
```

* With this fix:
```sql
SELECT COUNT(*) AS count_all, "books"."author_id" AS books_author_id
  FROM "books" INNER JOIN "photos" ON "photos"."book_id" = "books"."id"
 GROUP BY "books"."author_id"
```

0 files changed, 0 insertions, 0 deletions