ActionMailer https on URL with force_ssl = true

`config.force_ssl = true` will set
config.action_mailer.default_url_options = { protocol: 'https' }

If you have turned on force_ssl, and then gone to the effort of setting
a config.action_mailer.default_url_options = {host: 'example.com'} then
you are probably pointing people back to your current app and want
https on that too.
I would add `By default when <tt>config.force_ssl</tt> is set to true` to avoid any ambiguity :smile: