Enable warnings for mailer tests

actionmailer/Rakefile

@@ -24,7 +24,7 @@ Rake::TestTask.new { |t|
   t.libs << "test"
   t.pattern = 'test/*_test.rb'
   t.verbose = true
-  t.warning = false
+  t.warning = true
 }
 
 task :isolated_test do