Use original key format when displaying error for filter not allowed

Author: kevintraver

lib/jsonapi/request_parser.rb

@@ -384,7 +384,7 @@ def parse_filters(resource_klass, filters)
         if resource_klass._allowed_filter?(filter)
           parsed_filters[filter] = value
         else
-          fail JSONAPI::Exceptions::FilterNotAllowed.new(filter)
+          fail JSONAPI::Exceptions::FilterNotAllowed.new(key)
         end
       end