Pass on headers

[eelcoj]

Something like

class OrderEmail < Courrier::Email
  headers = {
    "List-Unsubscribe-Post": "List-Unsubscribe=One-Click" # coming from Postmark's docs: https://postmarkapp.com/support/article/1299-how-to-include-a-list-unsubscribe-header#post
  }

  def subject = "Here is your order!"

  def text
    <<~TEXT
      text body here
    TEXT
  end

  def html
    <<~HTML
      html body here
    HTML
  end
end

This should then merge with the provider-specific headers: https://github.com/Rails-Designer/courrier/blob/main/lib/courrier/email/providers/base.rb#L34