Set action_mailer.default_url_options values in development and test.

Prior to this commit, new Rails applications would raise
`ActionView::Template::Error` if a mailer included a url built with a
`*_path` helper.

Since we already know [new apps will be served on `localhost:3000`][new
apps], we set this as the value in `development`.

In an effort to remain consistent with existing patters, we set the
`host` to `www.example.com` in `test.

[new apps]: https://github.com/rails/rails/blob/47300002db11d67d7b35103f5c429dad7dacdacd/README.md?plain=1#L81

Author: stevepolitodesign

railties/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Set `action_mailer.default_url_options` values in `development` and `test`.
+
+    Prior to this commit, new Rails applications would raise `ActionView::Template::Error`
+    if a mailer included a url built with a `*_path` helper.
+
+    *Steve Polito*
+
 *   Introduce `Rails::Generators::Testing::Assertions#assert_initializer`
 
     Compliments the existing `initializer` generator action.

railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt

@@ -43,6 +43,8 @@ Rails.application.configure do
   config.action_mailer.raise_delivery_errors = false
 
   config.action_mailer.perform_caching = false
+
+  config.action_mailer.define_url_options = { host: "localhost", port: 3000 }
   <%- end -%>
 
   # Print deprecation notices to the Rails logger.

railties/lib/rails/generators/rails/app/templates/config/environments/test.rb.tt

@@ -44,6 +44,10 @@ Rails.application.configure do
   # ActionMailer::Base.deliveries array.
   config.action_mailer.delivery_method = :test
 
+  # Unlike controllers, the mailer instance doesn't have any context about the
+  # incoming request so you'll need to provide the :host parameter yourself.
+  config.action_mailer.default_url_options = { host: "www.example.com" }
+
   <%- end -%>
   # Print deprecation notices to the stderr.
   config.active_support.deprecation = :stderr