Fix PersistedModel._defineChangeModel

Correctly handle the case when the model is attached multiple times
during the lifecycle, this happens because `loopback.createModel`
always makes an attempt to auto-attach.

lib/persisted-model.js

@@ -1423,10 +1423,15 @@ module.exports = function(registry) {
 
     if (this.dataSource) {
       attachRelatedModels(this);
-    } else {
-      this.once('dataSourceAttached', attachRelatedModels);
     }
 
+    // We have to attach related model whenever the datasource changes,
+    // this is a workaround for autoAttach called by loopback.createModel
+    var self = this;
+    this.on('dataSourceAttached', function() {
+      attachRelatedModels(self);
+    });
+
     return this.Change;
 
     function attachRelatedModels(self) {