use English as default language at "test_reminders_should_not_include_closed_issues" of unit mailer test
On Rails 3.1.3, test fails and output is not English.
<pre>
Error: test_reminders_should_not_include_closed_issues(MailerTest)
ActiveRecord::RecordInvalid: Validation failed: Tracker non è incluso nella lista
test/unit/mailer_test.rb:459:in `test_reminders_should_not_include_closed_issues'
</pre>
