Update lib/rdoc/options.rb

Co-authored-by: Sutou Kouhei <kou@clear-code.com>

Author: hsbt

lib/rdoc/options.rb

@@ -600,7 +600,7 @@ def to_yaml(*options) # :nodoc:
       yaml[ivar] = instance_variable_get("@#{ivar}")
     end
 
-    if defined?(Psych)
+    if yaml.respond_to?(:to_yaml)
       yaml.to_yaml
     else
       RDoc.yaml_serializer.dump(yaml)