Fix default mailer class not resolving with autoloading in background jobs

The `Rodauth::Rails::Feature` class gets autoloaded only when the Rodauth
configuration is getting evaluated. In the web server, this happens immediately
before the request reaches the Rails router, because the Rodauth middleware
loads the Rodauth app, which in turn loads Rodauth configuration.

However, in a background job process, enqueued email deliveries will first
resolve the mailer class, and then the mailer will load the Rodauth
configuration. So, the job processor will attempt to resolve
`Rodauth::Rails::Feature::Email::Mailer`, which fails, because the `rails`
Rodauth feature hasn't yet been loaded.

The solution is simple: move the mailer class in a place where it can be
autoloaded without the feature being loaded. As a bonus, this results in a much
shorter `Rodauth::Rails::Mailer` class.

Fixes #337

Author: janko

lib/rodauth/rails.rb

@@ -10,6 +10,7 @@ class Error < StandardError
     # This allows avoiding loading Rodauth at boot time.
     autoload :App, "rodauth/rails/app"
     autoload :Auth, "rodauth/rails/auth"
+    autoload :Mailer, "rodauth/rails/mailer"
 
     @app = nil
     @middleware = true

lib/rodauth/rails/feature/email.rb

@@ -12,20 +12,13 @@ module Email
 
         # Create emails with ActionMailer which uses configured delivery method.
         def create_email_to(to, subject, body)
-          Mailer.create_email(to: to, from: email_from, subject: "#{email_subject_prefix}#{subject}", body: body)
+          Rodauth::Rails::Mailer.create_email(to: to, from: email_from, subject: "#{email_subject_prefix}#{subject}", body: body)
         end
 
         # Delivers the given email.
         def send_email(email)
           email.deliver_now
         end
-
-        # ActionMailer subclass for correct email delivering.
-        class Mailer < ActionMailer::Base
-          def create_email(options)
-            mail(options)
-          end
-        end
       end
     end
   end

lib/rodauth/rails/mailer.rb

@@ -0,0 +1,9 @@
+module Rodauth
+  module Rails
+    class Mailer < ActionMailer::Base
+      def create_email(options)
+        mail(options)
+      end
+    end
+  end
+end