Fix monetized_attributes class discrepancy

Suppose we have a model

```ruby
class Investment < ActiveRecord::Base
  monetize :value
  monetize :discounted_value
end
```

and a subclass

```ruby
class BadInvestment < Investment
end
```

When we check the monetized_attributes of both the `Product` and `SpecialProduct` we get seemingly the same result:

```ruby
Investment.monetized_attributes
# => {
  value: value_cents,
  discounted_value: discounted_value_cents,
}

BadInvestment.monetized_attributes
# => {
  value: value_cents,
  discounted_value: discounted_value_cents,
}
```

...but when we check the class of the `monetized_attributes` we can see that one is a `ActiveSupport::HashWithIndifferentAccess` while the other is a `Hash`.

This commit fixes the discrepancy, ensuring both are a `ActiveSupport::HashWithIndifferentAccess`.

lib/money-rails/active_record/monetizable.rb

@@ -10,7 +10,7 @@ class ReadOnlyCurrencyException < MoneyRails::Error; end
 
       module ClassMethods
         def monetized_attributes
-          monetized_attributes = @monetized_attributes || {}
+          monetized_attributes = @monetized_attributes || {}.with_indifferent_access
 
           if superclass.respond_to?(:monetized_attributes)
             monetized_attributes.merge(superclass.monetized_attributes)