Fix the ._get_db() attribute after a Document.switch_db()

Without this patch, I've got:

```
myobj._get_db()

<bound method TopLevelDocumentMetaclass._get_db of <class 'oneflow.core.models.nonrel.Article'>>

```

I need to `myobj._get_db()()` to get the database.

I felt this like a bug.

regards,

mongoengine/document.py

@@ -400,7 +400,7 @@ class Document(BaseDocument):
         """
         with switch_db(self.__class__, db_alias) as cls:
             collection = cls._get_collection()
-            db = cls._get_db
+            db = cls._get_db()
         self._get_collection = lambda: collection
         self._get_db = lambda: db
         self._collection = collection