Add spec for data migration and remove inline class definition
- Since the AddSigEncToHeadersToMetaDefaults not only changes the schema but also the data, we decided to have a test to ensure the behavior.
- The inline class definition that was used to reference the list class with a temporary class that decouples the migration from the concrete class implementation threw an rspec error. Therefore the inline definition was replaced.
@muri, sorry that this took so long!