Add reply_to support to DeliveryMethod

[benjaminhouy]

Summary

This PR adds support for the reply_to email header in the DeliveryMethod class. The Userlist Push API already supports the reply_to parameter, but the gem's DeliveryMethod wasn't serializing it from ActionMailer Mail objects.

Changes

1. Added reply_to serialization - The serialize method now includes reply_to: serialize_address(mail.reply_to)
2. Updated serialize_address method - Modified to return nil for empty/nil addresses instead of empty arrays, ensuring .compact works correctly
3. Added comprehensive tests - Two new test contexts verify:
   • reply_to is properly serialized when present
   • reply_to is omitted from payload when not present (via .compact)

Backward Compatibility

This change is fully backward compatible:

• Uses existing serialize_address method for consistency with to/from handling
• Relies on .compact to omit reply_to when not set
• All existing tests pass (212 examples, 0 failures)

Use Case

This enables contact forms and other transactional emails to specify a custom reply-to address, allowing recipients to reply directly to the original sender rather than the system email address.

Example usage:

mail(
  to: 'support@example.com',
  from: 'noreply@example.com',
  reply_to: 'user@example.com',  # Now properly sent to Userlist API
  subject: 'Contact Form Submission'
)

Testing

All 212 existing tests pass, plus 2 new tests specifically for reply_to functionality.

[benedikt]

Thank you, @benjaminhouy!