Allow destroying a non-persisted Paranoid object

Currently destruction is made via touching the paranoid column. This works unless the object being destroyed has not yet been persisted, such as when replacing the other end of a has_one relation. There is no need to mark it destroyed in that scenario because it will not be stored in database.
http://api.rubyonrails.org/v4.1.1/classes/ActiveRecord/Persistence.html#method-i-touch

Point 3:

Note that touch must be used on a persisted object, or else an ActiveRecordError will be thrown. For example:

```ruby
ball = Ball.new
ball.touch(:updated_at)   # => raises ActiveRecordError
```