Remove instance variables in has_filters spec

This was a false positive in Rubocop, but we can avoid it by using the
attribute reader method we've just added.

.rubocop.yml

@@ -360,7 +360,6 @@ RSpec/HookArgument:
 RSpec/InstanceVariable:
   Enabled: true
   Exclude:
-    - spec/controllers/concerns/has_filters_spec.rb
     - spec/controllers/concerns/has_orders_spec.rb
 
 RSpec/LetBeforeExamples:

spec/controllers/concerns/has_filters_spec.rb

@@ -6,7 +6,7 @@ describe HasFilters do
     has_filters ["all", "pending", "reviewed"], only: :index
 
     def index
-      render plain: "#{@current_filter} (#{@valid_filters.join(" ")})"
+      render plain: "#{current_filter} (#{valid_filters.join(" ")})"
     end
   end