improvements on how to get self link on pagination class

Author: bacarini

lib/active_model/serializer/adapter/json_api/pagination_links.rb

@@ -41,8 +41,7 @@ def pages_from
           end
 
           def url(options)
-            self_link = options.fetch(:links) {{}}
-            self_link.fetch(:self) {} ? options[:links][:self] : original_url
+            @url ||= options.fetch(:links, {}).fetch(:self, nil) || original_url
           end
 
           def original_url