Fix env deprecated in Rails 5.0

I found it at the very bottom of the `test-consumer-features` CI build
job. See: https://github.com/rails/rails/issues/23294.

app/controllers/metal_decorator.rb

@@ -1,6 +1,6 @@
 # For the API
 ActionController::Metal.class_eval do
   def spree_current_user
-    @spree_current_user ||= env['warden'].user
+    @spree_current_user ||= request.env['warden'].user
   end
 end