mark_as_read! fails with exception on Rails 6+ if Reader has default_scope defined

Hey guys,
 
We're having the following issue:
If the reader has `default_scope` defined, running `mark_as_read!` on readable will raise an exception:

```
ActiveRecord::RecordInvalid: Validation failed: Reader must exist
```

This is happening as `belongs_to` definition of the `ReadMark` needs to specify `optional: true` while it doesn't.

Thank you,
Kon.