patch method to bypass JsonGuardProvider's guardUpdate for primary key columns

When called with a payload consisting only of the primary key, __apply_changes will not call context guard's `guardUpdate`.

```python
# assuming MyClass has a primary key property pkey
myObj = dbsession.query(MyClass).get('key 1')
myObj.apply_changes(dict(pkey='key 2'), context=guardInstance)
```
