Use -W:strict_unused_block when running tests on Ruby 3.4+

No changes needed to the code.

Rakefile

@@ -20,7 +20,7 @@ end
 
 desc "Run specs"
 task :spec do
-  sh %{#{FileUtils::RUBY} #{"-w" if RUBY_VERSION >= '3'} spec/simple_mailer.rb}
+  sh %{#{FileUtils::RUBY} #{"-w" if RUBY_VERSION >= '3'} #{'-W:strict_unused_block' if RUBY_VERSION >= '3.4'} spec/simple_mailer.rb}
 end
 
 task :default => :spec