Feedback on url() "decoration" of File: implementation and testing.

[richardkmichael]

Feedback wanted on the implementation of:

1. The "decorator" idea to set the url properties of a file: https://github.com/richardkmichael/carrierwave-activerecord/blob/master/lib/carrierwave-activerecord/storage/storage_provider.rb#L31
2. The reflection on Rails to determine the URL: https://github.com/richardkmichael/carrierwave-activerecord/blob/master/lib/carrierwave-activerecord/storage/storage_provider.rb#L66
3. The mocking of Rails in the storage provider spec: https://github.com/richardkmichael/carrierwave-activerecord/blob/master/spec/lib/carrierwave-activerecord/storage/storage_provider_spec.rb#L44