STI will not pick up custom generator

This works:

```ruby
Foo < ActiveRecord::Base
generate_public_uid generator: PublicUid::Generators::HexStringSecureRandom.new(50)
end

foo = Foo.new
foo.save!
foo.public_uid.length # => 50
```

this wil not

```class

```ruby
Bar 8
```

due to https://github.com/equivalent/public_uid/blob/master/lib/public_uid/model.rb#L25 instance method set the config is not copied across inheritance which is bad in this case

possible solution: use class variables `@@generator =`