Improve and finish the fallback to the In-Reply-To and References header for the reply-by-email feature

A few things to note:
- The IncomingEmail feature is now enabled even without a
  correctly-formatted sub-address
- Message-ID for new thread mail are kept the same so that subsequent
  notifications to this thread are grouped in the thread by the email
  service that receives the notification
  (i.e. In-Reply-To of the answer == Message-ID of the first thread message)
- To maximize our chance to be able to retrieve the reply key, we look
  for it in the In-Reply-To header and the References header
- The pattern for the fallback reply message id is "reply-[key]@[gitlab_host]"
- Improve docs thanks to Axil

1 files changed, 8 insertions, 18 deletions

diff --git a/spec/lib/gitlab/incoming_email_spec.rb b/spec/lib/gitlab/incoming_email_spec.rb
index bcdba8d4c12..afb3e26f8fb 100644
--- a/spec/lib/gitlab/incoming_email_spec.rb
+++ b/spec/lib/gitlab/incoming_email_spec.rb
@@ -7,24 +7,8 @@ describe Gitlab::IncomingEmail, lib: true do
         stub_incoming_email_setting(enabled: true)
       end
 
-      context "when the address is valid" do
-        before do
-          stub_incoming_email_setting(address: "replies+%{key}@example.com")
-        end
-
-        it "returns true" do
-          expect(described_class.enabled?).to be_truthy
-        end
-      end
-
-      context "when the address is invalid" do
-        before do
-          stub_incoming_email_setting(address: "replies@example.com")
-        end
-
-        it "returns false" do
-          expect(described_class.enabled?).to be_falsey
-        end
+      it 'returns true' do
+        expect(described_class.enabled?).to be_truthy
       end
     end
 
@@ -58,4 +42,10 @@ describe Gitlab::IncomingEmail, lib: true do
       expect(described_class.key_from_address("replies+key@example.com")).to eq("key")
     end
   end
+
+  context 'self.key_from_fallback_reply_message_id' do
+    it 'returns reply key' do
+      expect(described_class.key_from_fallback_reply_message_id('reply-key@localhost')).to eq('key')
+    end
+  end
 end