fix: Use to_i in campaign scheduled_at key (#3968)

spec/controllers/api/v1/accounts/campaigns_controller_spec.rb

@@ -145,6 +145,7 @@ RSpec.describe 'Campaigns API', type: :request do
         response_data = JSON.parse(response.body, symbolize_names: true)
         expect(response_data[:campaign_type]).to eq('one_off')
         expect(response_data[:scheduled_at].present?).to eq true
+        expect(response_data[:scheduled_at]).to eq(2.days.from_now.to_i)
         expect(response_data[:audience].pluck(:id)).to include(label1.id, label2.id)
       end
     end