Fixes issue where only id would be saved when updating an existing object for mongodb driver.

.save instead of .update was being called which would persist only a
data object with the id value to the database on doing a save of an
existing object.

lib/adapters/mongodb.js

@@ -63,7 +63,7 @@ MongoDB.prototype.create = function (model, data, callback) {
 };
 
 MongoDB.prototype.save = function (model, data, callback) {
-    this.collection(model).save({_id: new ObjectID(data.id)}, data, function (err) {
+    this.collection(model).update({_id: new ObjectID(data.id)}, data, function (err) {
         callback(err);
     });
 };