Removed empty translation entries.

We should actually look into doing a comprehensive audit of translation keys before going to Spree 2.0 to remove anything that is no longer needed.  For instance, do we still need the translations related to product scopes/groups?
[api] Allow especially overriden attribute setters for line items

Fixes #2916
Revert "[api] Remove LineItemDecorator"

This reverts commit aff30f52f3dad3f8aca9ed838575542a30c9a1e6.

Turns out that this is required if we are going to use :as => :api. Whodathunk?
Ensure currency option is preserved by OrderContents class