Use marginally more efficient call to fetch by moving default to a block

Author: chrisbranson

lib/active_model/serializer/adapter.rb

@@ -75,7 +75,7 @@ def meta_key
       end
 
       def root
-        @options.fetch(:root, serializer.json_key)
+        @options.fetch(:root) { serializer.json_key }
       end
 
       def include_meta(json)