not use assert_not_nil in Errors#[]

r7593 etc. replaced Rails2 Errors#on.
Rails3 Errors#[] always return array.
So, Rails3 Errors#[] is always not nil.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12070 e93f8b46-1217-0410-a6f0-8f06a7374b81

test/unit/user_test.rb

@@ -401,7 +401,7 @@ class UserTest < ActiveSupport::TestCase
     u = User.new
     u.mail_notification = 'foo'
     u.save
-    assert_not_nil u.errors[:mail_notification]
+    assert_not_equal [], u.errors[:mail_notification]
   end
 
   def test_password