Dont allow serialization of firestore settings

When logging any firestore object like WriteBatch,Transaction,etc the settings object also gets logged / exposed
This can be seen by running JSON.stringify on any firestore object even a document reference
Many developers log firestore objects to help them debug testing/prod issues, this leaking of entire firestore key via this._settings is a bad practice as per me
We can also use Object.defineProperty to make it non-enumerable or any other technique that you like

dev/src/index.ts

@@ -652,6 +652,7 @@ export class Firestore implements firestore.Firestore {
     }
 
     this._settings = settings;
+    this._settings.toJson = function(){ return undefined; }
     this._serializer = new Serializer(this);
   }