use assert_select instead of include? at "test_register" of unit mailer test

On Rails 3.0.11, token breaks with new line.
So, test fails.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@8916 e93f8b46-1217-0410-a6f0-8f06a7374b81