Make enable_starttls_auto opt-in in ActionMailer

This commit:

http://github.com/rails/rails/commit/732c724df61bc8b780dc42817625b25a321908e4

Enabled automatically STARTTLS in SMPT servers making some
applications break.

I've created a patch that makes enable_starttls_auto opt-in. Updated
documentation and tests.

http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/1731-make-enable_starttls_auto-opt-in-in-actionmailer