Use serializable_hash for compared results

app/controllers/projects/merge_requests_controller.rb

@@ -108,7 +108,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     when :parsing
       render json: '', status: :no_content
     when :parsed
-      render json: result[:data], status: :ok
+      render json: result[:data].to_json, status: :ok
     when :error
       render json: { status_reason: result[:status_reason] }, status: :bad_request
     else

app/services/ci/compare_test_reports_service.rb

@@ -14,7 +14,7 @@ module Ci
           status: :parsed,
           data: TestReportsComparerSerializer
             .new(project: project)
-            .represent(comparer).to_json
+            .represent(comparer).as_json
         }
       rescue => e
         { status: :error, status_reason: e.message }