involve sti_name and find_sti_class in subclass_from_attrs method

This is needed when parent class override `find_sti_class` and `sti_name` methods

```ruby
class Identity < ActiveRecord::Base
  self.inheritance_column = :provider
  
  class << self
    def find_sti_class(type_name)
      ActiveSupport::Dependencies.constantize "Identity::#{type_name.titleize}"
    end
    
    def sti_name
      name.demodulize.downcase
    end
  end
end

class Identity::Facebook < Identity
end

class Identity::Twitter < Identity
end
```

Without this fix `Identity.new provider: 'facebook'` raises "ActiveRecord::SubclassNotFound: Invalid single-table inheritance type: facebook is not a subclass of Identity" error