avoiding nil cases when dynamically creating a class

Author: joaomdmoura

lib/active_model/serializer/adapter.rb

@@ -83,7 +83,7 @@ def meta_key
       end
 
       def root
-        serializer.json_key.to_sym
+        serializer.json_key.to_sym if serializer.json_key
       end
 
       def include_meta(json)