Add `to_json` to `represent` method call

app/controllers/concerns/boards_responses.rb

@@ -69,7 +69,7 @@ module BoardsResponses
   end
 
   def serialize_as_json(resource)
-    serializer.represent(resource).to_json
+    serializer.represent(resource).as_json
   end
 
   def respond_with(resource)

changelogs/unreleased/3861-use-serializers-for-project-group-boards.yml

----
-title: Refactor Board JSON serialization to use Grape::Entity instead of manual serialization
-  (to_json)
-merge_request:
-author:
-type: changed

spec/controllers/concerns/boards_responses_spec.rb

@@ -17,7 +17,7 @@ describe BoardsResponses do
     it 'serializes properly' do
       expected = { "id" => board.id }
 
-      expect(JSON.parse(subject.serialize_as_json(board))).to include(expected)
+      expect(subject.serialize_as_json(board)).to include(expected)
     end
   end
 end