Add controller specs, fix bad attributes in ProductPackagesController

In my earlier PR I botched the attributes in the ProductPackagesController.  This PR:

1. Corrects the erroneous attribute list
2. Adds a basic controller spec for Spree::Admin::ProductPackagesController to cover the issue, as well as a basic spec for index
3. Adds a basic controller spec for Spree::Admin::ActiveShippingSettingsController
4. Uses Spree.t for a lingering i18n reference.