Add reply_to support to DeliveryMethod

The Userlist Push API supports the reply_to parameter for messages,
but the DeliveryMethod class wasn't serializing it from ActionMailer
Mail objects.

This commit:
- Adds reply_to to the serialize method
- Updates serialize_address to return nil for empty addresses
- Adds comprehensive tests for both presence and absence of reply_to
- Maintains backward compatibility via .compact

All existing tests pass (212 examples, 0 failures).

Author: benjaminhouy

lib/userlist/delivery_method.rb

@@ -19,12 +19,15 @@ def serialize(mail)
     {
       to: serialize_address(mail.to),
       from: serialize_address(mail.from),
+      reply_to: serialize_address(mail.reply_to),
       subject: mail.subject,
       body: serialize_body(mail.body)
     }.compact
   end
 
   def serialize_address(address)
+    return if address.nil? || Array(address).empty?
+
     Array(address).map(&:to_s)
   end
 

spec/userlist/delivery_method_spec.rb

@@ -104,5 +104,55 @@
         delivery_method.deliver!(mail)
       end
     end
+
+    context 'with reply_to address' do
+      let(:mail) do
+        Mail.new(
+          to: 'user@example.com',
+          from: 'sender@example.com',
+          reply_to: 'reply@example.com',
+          subject: 'Test Subject',
+          body: 'Hello world'
+        )
+      end
+
+      it 'includes reply_to in the payload' do
+        expected_payload = {
+          to: ['user@example.com'],
+          from: ['sender@example.com'],
+          reply_to: ['reply@example.com'],
+          subject: 'Test Subject',
+          body: { type: :text, content: 'Hello world' },
+          theme: nil
+        }
+
+        expect(messages).to receive(:push).with(expected_payload)
+        delivery_method.deliver!(mail)
+      end
+    end
+
+    context 'without reply_to address' do
+      let(:mail) do
+        Mail.new(
+          to: 'user@example.com',
+          from: 'sender@example.com',
+          subject: 'Test Subject',
+          body: 'Hello world'
+        )
+      end
+
+      it 'omits reply_to from the payload' do
+        expected_payload = {
+          to: ['user@example.com'],
+          from: ['sender@example.com'],
+          subject: 'Test Subject',
+          body: { type: :text, content: 'Hello world' },
+          theme: nil
+        }
+
+        expect(messages).to receive(:push).with(expected_payload)
+        delivery_method.deliver!(mail)
+      end
+    end
   end
 end