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.
Also we should not change the object in place here, we might be changing user's config this way.