Fix regression on .select method

This was a common pattern:
query = author.posts.select(:title)
connection.select_one(query)

However .select returns a ActiveRecord::AssociationRelation, which has
the bind information, so we can use that to get the right sql query.

[fixes #7538]
[related #13731]
Add more test for select* methods

- Use Relation instead of AssociationRelation
- Fix select_values for mysql