FIX: email receiver should ignore x-auto-response-suppress

This header is used by Microsoft Exchange to indicate when certain types of
autoresponses should not be generated for an email.

It triggers our "is this mail autogenerated?" detection, but should not be used
for this purpose.

lib/email/receiver.rb

@@ -383,7 +383,7 @@ module Email
         @mail[:subject].to_s[
           /\A\s*(Auto:|Automatic reply|Autosvar|Automatisk svar|Automatisch antwoord|Abwesenheitsnotiz|Risposta Non al computer|Automatisch antwoord|Auto Response|Respuesta automática|Fuori sede|Out of Office|Frånvaro|Réponse automatique)/i
         ] ||
-        @mail.header.to_s[
+        @mail.header.reject { |h| h.name.downcase == "x-auto-response-suppress" }.to_s[
           /auto[\-_]?(response|submitted|replied|reply|generated|respond)|holidayreply|machinegenerated/i
         ]
     end

spec/fixtures/emails/quirks_exchange_xars.eml

@@ -0,0 +1,10 @@
+Return-Path: <discourse@bar.com>
+From: Foo Bar <discourse@bar.com>
+To: reply+4f97315cc828096c9cb34c6f1a0d6fe8@bar.com
+Date: Fri, 15 Jan 2016 00:12:43 +0100
+Message-ID: <21@foo.bar.mail>
+Mime-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+X-Auto-Response-Suppress: DR, RN, NRN, OOF, AutoReply
+
+Some thing.

spec/lib/email/receiver_spec.rb

@@ -521,6 +521,10 @@ RSpec.describe Email::Receiver do
       expect { process(:invalid_from_2) }.to raise_error(Email::Receiver::NoSenderDetectedError)
     end
 
+    it "doesn't raise an AutoGeneratedEmailError due to an X-Auto-Response-Suppress header" do
+      expect { process(:quirks_exchange_xars) }.to change { topic.posts.count }
+    end
+
     it "doesn't raise an AutoGeneratedEmailError when the mail is auto generated but is allowlisted" do
       SiteSetting.auto_generated_allowlist = "foo@bar.com|discourse@bar.com"
       expect { process(:auto_generated_allowlisted) }.to change { topic.posts.count }