ActiveRecord::Enum, model.status_was return a String value, not integer

```ruby
class Conversation < ActiveRecord::Base
  enum status: { active: 0, archived: 1 }
end
```

Before:

```ruby
conversation.status = :active
conversation.status = :archived
conversation.status_was # => 0
```

After:

```ruby
conversation.status_was # => "active"
```