Send newsletter emails in order

The list of emails for sending newsletters will always be ordered by
user creation date, so it will be easier to debug and to know for
which users the email has been sent.

spec/lib/user_segments_spec.rb

@@ -190,4 +190,15 @@ describe UserSegments do
     end
   end
 
+  describe "#user_segment_emails" do
+    it "returns list of emails sorted by user creation date" do
+      create(:user, email: "first@email.com", created_at: 1.day.ago)
+      create(:user, email: "last@email.com")
+
+      emails = described_class.user_segment_emails(:all_users)
+      expect(emails.first).to eq "first@email.com"
+      expect(emails.last).to eq "last@email.com"
+    end
+  end
+
 end