Adds implicit conversion of regclass to text

In the following example

```ruby
class Transaction < ActiveRecord::Base
  belongs_to :account
  scope :with_type, -> do 
    select "#{self.name.underscore.pluralize}.*, #{self.name.underscore.pluralize}.tableoid::regclass as table_name"
  end
end
```

Calling `Transaction.with_type` will generate a warning like:
> unknown OID: table_name(2205) (SELECT  game_transactions.*, game_transactions.tableoid::regclass as table_name FROM "game_transactions"  WHERE "game_transactions"."account_id" = $1 AND "game_transactions"."id" = 226 LIMIT 1)

This commit ensures that we implicitly just convert the regclass to text to get rid of this warning.