more expansions for API message: activity_entries (#344)

api/structures/message_api_structure.rb

@@ -56,4 +56,11 @@ structure :message do
   expansion(:headers) { o.headers }
   expansion(:raw_message) { Base64.encode64(o.raw_message) }
+  
+  expansion(:activity_entries) {
+    {
+      :loads => o.loads,
+      :clicks => o.clicks
+    }
+  }
 end