Extend Mail::Message

We want to store the tempalte id, personalisation, reference and reply
to id options inside the `message` object, right now these are added to
the header of the message, which feels a little clunky.

Here we use the existing module to add the attributes to message so we
can set and get the values.

We also drop the `preview` method, this method hides the fact that the
impelmentation of the `preview` method is in the DeliveryMethod - on
balance the extra code to call the method seems worth the price of the
understanding where messages are being sent - to be honest, it already
feels odd that the message calls a method and passes it self - but that
might just be me?

lib/mail/notify/message.rb

@@ -3,9 +3,7 @@
 module Mail
   module Notify
     module Message
-      def preview
-        delivery_method.preview(self) if delivery_method.respond_to?(:preview)
-      end
+      attr_accessor :template_id, :personalisation, :reply_to_id, :reference
     end
   end
 end

spec/mail/notify/message_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+class FakeTestClass
+  include Mail::Notify::Message
+end
+
+RSpec.describe Mail::Notify::Message do
+  subject { FakeTestClass.new }
+
+  it "adds a template_id attribute" do
+    expect(subject.template_id = "template-id").to eql "template-id"
+    expect(subject.template_id).to eql "template-id"
+  end
+
+  it "adds a personalisation attribute" do
+    expect(subject.personalisation = {name: "Test Name"}).to eql({name: "Test Name"})
+    expect(subject.personalisation).to eql({name: "Test Name"})
+  end
+
+  it "adds the reply_to_id attribute" do
+    expect(subject.reply_to_id = "test-reply-to-id").to eql("test-reply-to-id")
+    expect(subject.reply_to_id).to eql("test-reply-to-id")
+  end
+
+  it "adds the reference attribute" do
+    expect(subject.reference = "test-reference").to eql("test-reference")
+    expect(subject.reference).to eql("test-reference")
+  end
+end