Eliminates `unknown OID: post_title(705)` message with PostgreSQL

This pull request eliminates `unknown OID: post_title(705)` message when tested with PostgreSQL adapter. 

```ruby
$ ARCONN=postgresql ruby -Itest test/cases/relation_test.rb -n test_respond_to_for_non_selected_element
Using postgresql
Run options: -n test_respond_to_for_non_selected_element --seed 34103

# Running tests:

unknown OID: post_title(705) (SELECT  'title' as post_title FROM "posts"   ORDER BY "posts"."id" ASC LIMIT 1)
.

Finished tests in 0.147927s, 6.7601 tests/s, 13.5202 assertions/s.

1 tests, 2 assertions, 0 failures, 0 errors, 0 skips
$
```