add spec and code to include guid in comment's export

#6115

app/serializers/export/comment_serializer.rb

@@ -1,6 +1,7 @@
 module Export
   class CommentSerializer < ActiveModel::Serializer
-    attributes :text,
+    attributes :guid,
+               :text,
                :post_guid
 
     def post_guid

spec/serializers/comment_serializer_spec.rb

@@ -0,0 +1,10 @@
+require "spec_helper"
+
+describe Export::CommentSerializer do
+  let(:comment) { create(:comment) }
+  subject(:json_output) { Export::CommentSerializer.new(comment).to_json }
+
+  it { is_expected.to include("\"guid\":\"#{comment.guid}\"") }
+  it { is_expected.to include("\"post_guid\":\"#{comment.post.guid}\"") }
+  it { is_expected.to include("\"text\":\"#{comment.text}\"") }
+end