Payment method preferences returns different object types depending on setup

[seand7565]

When setting up a payment_method, if you don't use static model preferences, you get a Hash object when you call payment_method.preferences. If you do use static model preferences, you instead get a Spree::Preferences::StaticModelPreferences::Definition object when you call payment_method.preferences

While using the default preference source, this command will succeed:
payment_method.preferences[:available_to_users]
While using static model preferences, the same command will fail.

Solidus Version:
2.10 - though likely an issue in more versions

To Reproduce

1. Create a Check payment method
2. Set it to use a static model preference (example provided below)
3. Open a console, run Spree::PaymentMethod.last.preferences[:test_mode] - it will fail
4. Set the payment method to use the "custom" preference source
5. In the console, run Spree::PaymentMethod.last.preferences[:test_mode] - it will succeed

Example static model preference:

# config/initializers/example_credentials.rb

Spree::Config.configure do |config|
  config.static_model_preferences.add(
    Spree::PaymentMethod::Check,
    'example_credential_source', {
      test_mode: false
    }
  )
end

Current behavior
payment_method.preferences could return either a Hash or a model object depending on the setup

Expected behavior
payment_method.preferences should always return just one object type - IMO preferably a Hash object.

Screenshots

[peterberkenbosch]

Using preferred_ prefix for the preferences works on both setups. I do agree the hash option is more Ruby idiomatic :)