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

app/views/api/v1/models/_campaign.json.jbuilder

@@ -13,7 +13,7 @@ json.campaign_status resource.campaign_status
 json.enabled resource.enabled
 json.campaign_type resource.campaign_type
 if resource.campaign_type == 'one_off'
-  json.scheduled_at resource.scheduled_at
+  json.scheduled_at resource.scheduled_at.to_i
   json.audience resource.audience
 end
 json.trigger_rules resource.trigger_rules

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