Fix for improperly namespaced url in admin

Without this change controllers which are:
a) outside of the Spree module namespace and
b) inherit from Spree::Admin::BaseController and
c) are routed inside the application's routing, not Spree's

will not be able to determine the url and their views will not
compile.

Fixes #2500
[api] No need to convert template name to symbol when rendering RablTemplate