FIX: Use our header value instead of custom header on duplicates (#16711)

When we build and send emails using MessageBuilder and Email::Sender
we add custom headers defined in SiteSetting.email_custom_headers.
However this was causing errors in cases where the custom headers
defined a header that we already specify in outbound emails (e.g.
the Precedence: list header for topic/post emails).

This commit makes it so we always use the header value defined in Discourse
core if there is a duplicate, discarding the custom header value
from the site setting.

cf. https://meta.discourse.org/t/email-notifications-fail-if-duplicate-headers-exist/222960/14

spec/integration/email_outbound_spec.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+# We already have Email::Sender and Email::MessageBuilder specs along
+# with mailer specific mailer specs like UserEmail, but sometimes we need
+# to test things along the whole outbound flow including the MessageBuilder
+# and the Sender.
+describe "Outbound Email" do
+  def send_email(opts = {})
+    message = TestMailer.send_test("test@test.com", opts)
+    result = Email::Sender.new(message, :test_message).send
+    [message, result]
+  end
+
+  context "email custom headers" do
+    it "discards the custom header if it is one that has already been set based on arguments" do
+      SiteSetting.email_custom_headers = "Precedence: bulk"
+      post = Fabricate(:post)
+      message, result = send_email(post_id: post.id, topic_id: post.topic_id)
+      expect(message.header["Precedence"].value).to eq("list")
+    end
+
+    it "does send unique custom headers" do
+      SiteSetting.email_custom_headers = "SuperUrgent: wow-cool"
+      post = Fabricate(:post)
+      message, result = send_email(post_id: post.id, topic_id: post.topic_id)
+      expect(message.header["SuperUrgent"].value).to eq("wow-cool")
+    end
+  end
+end