Fix the ._get_db() attribute after a Document.switch_db()
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,
gavincabbage/mongoengine,gmacon/mongoengine,reallistic/mongoengine,9nix00/mongoengine,blaggacao/mongoengine,RussellLuo/mongoengine,ConsumerPhysics/mongoengine,larsbutler/mongoengine,elephanter/mongoengine,Davidrjx/mongoengine,MongoEngine/mongoengine,iandd0824/mongoengine,prijindal/mongoengine,WingGao/mongoengine,jimmyshen/mongoengine,MRigal/mongoengine,cogniteev/mongoengine,ProgressivePlanning/mongoengine,cogniteev/mongoengine,ms5/mongoengine,sourcelair/mongoengine,khallaghi/mongoengine,nicolascine/mongoengine,eli-b/mongoengine,Catstyle/mongoengine,MakerReduxCorp/mongoengine,Davidrjx/mongoengine,noirbizarre/mongoengine,touilleMan/mongoengine,DavidBord/mongoengine,losintikfos/mongoengine,Multiposting/mongoengine,yarneo/mongoengine,starsirius/mongoengine,nathanial/mongoengine,MakerReduxCorp/mongoengine,muhammad-ammar/mongoengine