Test extensions to Mail gem

The from_address method is added to Mail::Message, but is not used
internally to ActionMailbox, nor tested (even implicitly). As it is part
of the public API, it feels important to at least have a basic test of
it.

Similarly, the x_original_to_addresses was only implicitly tested via
the recipients_addresses method. Given other similar methods are
explicitly tested, it feels like an oversight not to test it as well.

As the test introduced from_address didn't align with the naming of
RecipientsTest, I renamed it to the more generic AddressesTest.

actionmailbox/test/unit/mail_ext/addresses_test.rb

@@ -3,15 +3,20 @@
 require_relative "../../test_helper"
 
 module MailExt
-  class RecipientsTest < ActiveSupport::TestCase
+  class AddressesTest < ActiveSupport::TestCase
     setup do
       @mail = Mail.new \
+        from: "sally@example.com",
         to: "david@basecamp.com",
         cc: "jason@basecamp.com",
         bcc: "andrea@basecamp.com",
         x_original_to: "ryan@basecamp.com"
     end
 
+    test "from address uses address object" do
+      assert_equal "example.com", @mail.from_address.domain
+    end
+
     test "recipients include everyone from to, cc, bcc, and x-original-to" do
       assert_equal %w[ david@basecamp.com jason@basecamp.com andrea@basecamp.com ryan@basecamp.com ], @mail.recipients
     end
@@ -31,5 +36,9 @@ class RecipientsTest < ActiveSupport::TestCase
     test "bcc addresses use address objects" do
       assert_equal "basecamp.com", @mail.bcc_addresses.first.domain
     end
+
+    test "x_original_to addresses use address objects" do
+      assert_equal "basecamp.com", @mail.x_original_to_addresses.first.domain
+    end
   end
 end