Fix: view_mail does not duplicate headers

With a view mail we have to call `mail` twice, once to get the body of
the email from the view and once again to override the format contents.

We were passing in any custom headers for both calls which was resulting
in duplicate headers in the message.

By calling `mail` without the headers, we can get the body and not
modify the headers, leaving that for the second call.

Interestingly adding custom headers is pointless, as they will never end
up in the email from Notify - we may want to stop messing with the
headers at all - I think it is a hangover from the v1 implementation.

This was kindly identified by @inulty-dfe in #162 who are testing the
header Mail::Fields that are potentially duplicated.

CHANGELOG.md

@@ -8,6 +8,8 @@ The format is based on [Keep a Changelog]
 
 - Fixed bug in `view_mail` and `template_mail` that meant the options were being
   included in the preview email headers for no reason - thanks to @inulty-dfe
+- Fixed a bug in `view_mail` that meant custom headers in the preview email were
+  duplicated - thanks to @inulty-dfe
 
 ## [2.0.0] - 2024-04-01
 

lib/mail/notify/mailer.rb

@@ -76,9 +76,12 @@ def view_mail(template_id, options)
         subject = options[:subject]
         headers = options.except(:personalisation, :reply_to_id, :reference)
 
-        # we have to render the view for the message and grab the raw source, then we set that as the
+        # We have to render the view for the message and grab the raw source, then we set that as the
         # body in the personalisation for sending to the Notify API.
-        body = mail(headers).body.raw_source
+        #
+        # We do not pass the headers as the call to `mail` will keep adding headers resulting in
+        # duplication when we have to call it again later.
+        body = mail.body.raw_source
 
         # The 'view mail' works by sending a subject and body as personalisation options, these are
         # then used in the Notify template to provide content.

spec/mail/notify/mailer_spec.rb

@@ -132,6 +132,20 @@
       expect(message.header[:reply_to_id]).to be_nil
       expect(message.header[:reference]).to be_nil
     end
+
+    it "sets custom headers only once" do
+      message_params = {
+        template_id: "template-id",
+        to: "[email protected]",
+        subject: "Test subject",
+        custom_header: "custom header value"
+      }
+
+      message = TestMailer.with(message_params).test_view_mail
+
+      expect(message.header["custom-header"]).to be_a(Mail::Field)
+      expect(message.header["custom-header"].value).to eq("custom header value")
+    end
   end
 
   describe "#template_email" do
@@ -236,6 +250,20 @@
       expect(message.header[:reply_to_id]).to be_nil
       expect(message.header[:reference]).to be_nil
     end
+
+    it "sets custom headers only once" do
+      message_params = {
+        template_id: "template-id",
+        to: "[email protected]",
+        subject: "Test subject",
+        custom_header: "custom header value"
+      }
+
+      message = TestMailer.with(message_params).test_view_mail
+
+      expect(message.header["custom-header"]).to be_a(Mail::Field)
+      expect(message.header["custom-header"].value).to eq("custom header value")
+    end
   end
 
   describe "#blank_allowed" do