fix: updating message_annotations type

Author: magne

lib/message.js

@@ -287,7 +287,7 @@ define_outcome({
     fields:[
         {name:'delivery_failed', type:'boolean'},
         {name:'undeliverable_here', type:'boolean'},
-        {name:'message_annotations', type:'map'}
+        {name:'message_annotations', type:'symbolic_map'}
     ]});
 
 module.exports = message;