Since they are nodoc there is no need to be protected.

We are already filtering the keys in the assign_headers_to_message
method so we can just update the filter.

This reverts commit 65a61ab7, reversing
changes made to 14314ca1.

This PR broke the build

this allows us to construct mailer objects without possibly disastrous
side-effects.

`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
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.

Contrary to what the name suggests, PLAIN SMTP authentication is not
sent in plain text but is Base64 encoded like the LOGIN method. Their
difference is described in the third link below.

* https://tools.ietf.org/html/rfc4954
* https://en.wikipedia.org/wiki/SMTP_Authentication
* http://www.samlogic.net/articles/smtp-commands-reference-auth.htm

One or multiple mail interceptors can be unregistered using
`ActionMailer::Base.unregister_interceptors` or
`ActionMailer::Base.unregister_interceptor`.

For preview interceptors, it's possible to use
`ActionMailer::Base.unregister_preview_interceptors` or
`ActionMailer::Base.unregister_preview_interceptor`.

Refactors logic to constantize a string/symbol into separate method.

Given the following templates:

    mailer/demo.html.erb
    mailer/demo.en.html.erb
    mailer/demo.pt.html.erb

Before this change for a locale that doesn't have its related file
the `mailer/demo.html.erb` will
be rendered even if `en` is the default locale.

Now `mailer/demo.en.html.erb` has precedence over the file without
 locale.

Also, it is possible to give a fallback.

    mailer/demo.pt.html.erb
    mailer/demo.pt-BR.html.erb

So if the locale is `pt-PT`, `mailer/demo.pt.html.erb` will be
 rendered given the right I18n fallback configuration.

Fixes #11884.

Just a little syntax error I spotted by accident. Cannot pass hash with curly braces without normal parentheses.

Since some headers can appear multiple times in an email it's required
to set it to nil first when you want to overwrite an existing one.

This commit add some information about this process.

Fix #15912