mailer.rb - Fix subject to support custom notifications or basic status.

This was broken as part of this previous commit: https://github.com/sensu/sensu-community-plugins/commit/18b55f933b54e0d814216c7484a9ef203c91e841
Previously if you had a custom notification defined in your check the subject would include the message.  With the above commit that's now replaced with a simple OK, WARNING, CRITICAL.  So this PR restores the original functionality along with supporting the basic status if there is no notification message set.