Fix method redefined warning

Author: bf4

test/generators/serializer_generator_test.rb

@@ -67,6 +67,7 @@ def stub_safe_constantize(expected:)
     yield
   ensure
     String.class_eval do
+      undef_method :safe_constantize
       alias_method :safe_constantize, :old
       undef_method :old
     end