Check if key is present with string not symbol

closes gitlab-org/gitlab-ce#51225

spec/controllers/projects/jobs_controller_spec.rb

@@ -166,8 +166,8 @@ describe Projects::JobsController, :clean_gitlab_redis_shared_state do
             expect(response).to match_response_schema('job/job_details')
             expect(json_response['artifact']['download_path']).to match(%r{artifacts/download})
             expect(json_response['artifact']['browse_path']).to match(%r{artifacts/browse})
-            expect(json_response['artifact']).not_to have_key(:expired)
-            expect(json_response['artifact']).not_to have_key(:expired_at)
+            expect(json_response['artifact']).not_to have_key('expired')
+            expect(json_response['artifact']).not_to have_key('expired_at')
           end
         end
 
@@ -177,8 +177,8 @@ describe Projects::JobsController, :clean_gitlab_redis_shared_state do
           it 'exposes needed information' do
             expect(response).to have_gitlab_http_status(:ok)
             expect(response).to match_response_schema('job/job_details')
-            expect(json_response['artifact']).not_to have_key(:download_path)
-            expect(json_response['artifact']).not_to have_key(:browse_path)
+            expect(json_response['artifact']).not_to have_key('download_path')
+            expect(json_response['artifact']).not_to have_key('browse_path')
             expect(json_response['artifact']['expired']).to eq(true)
             expect(json_response['artifact']['expire_at']).not_to be_empty
           end