Fix: Don't show non-members as readers when the post is a whisper

spec/requests/post_readers_controller_spec.rb

@@ -76,6 +76,18 @@ describe PostReadersController do
 
         expect(readers).to be_empty
       end
+
+      it 'xxxxx' do
+        @post.update(post_type: Post.types[:whisper])
+        non_member_reader = Fabricate(:user)
+        @group_message.allowed_users << non_member_reader
+        TopicUser.create!(user: non_member_reader, topic: @group_message, last_read_post_number: 4)
+
+        get '/post_readers.json', params: { id: @post.id }
+        readers = JSON.parse(response.body)['post_readers']
+
+        expect(readers).to be_empty
+      end
     end
 
     def assert_reader_is_correctly_serialized(reader_data, reader, post)