Fixed model attribute synchronization for update method

closes bookshelf#2007

- The attributes object can be modified within `triggerThen` through hooks. Sample scenario described here bookshelf#2007 (comment).
- Recalculating 'attributesToSave' object in after-triggerThen step solves the syncronization issue because it is still based on model's `attributes` property (the reference is not lost)

lib/model.js

@@ -974,7 +974,9 @@ const BookshelfModel = ModelBase.extend(
             Helpers.saveConstraints(this, this.relatedData);
           }
 
-          const attributesToSave = method === 'update' && options.patch ? attrs : this.attributes;
+          const getAttributesToSave = function(method, options, model) {
+            return method === 'update' && options.patch ? attrs : model.attributes;
+          };
 
           // Gives access to the `query` object in the `options`, in case we need it
           // in any event handlers.
@@ -1039,12 +1041,12 @@ const BookshelfModel = ModelBase.extend(
           return this.triggerThen(
             method === 'insert' ? 'saving creating' : 'saving updating',
             this,
-            attributesToSave,
+            getAttributesToSave(method, options, this),
             options
           )
             .bind(this)
             .then(function() {
-              return sync[options.method](attributesToSave);
+              return sync[options.method](getAttributesToSave(method, options, this));
             })
             .then(function(resp) {
               // After a successful database save, the id is updated if the model was created