Fix: auth db defaults

[playerx]

Current Behaviour:
authSource is changed when database name is passed in the connection string. The issue is in setting defaults, same connection string worked on nodejs official driver but didn't work with deno_mongo.

After Merge:
authSource will be changed only if authSource will be passed in the connection string, otherwise it will keep the default admin db as authSource

Related issue:
#319

[erfanium]

this example from node's official driver is against this change:

import { MongoClient, ObjectId } from "mongodb";

const client = new MongoClient(
  "mongodb+srv://app:myapppass@cluster0.kaxqc.mongodb.net/dbName"
);
console.log(client.dbName);

[playerx]

I've just tried same connection string with nodejs mongodb:

mongodb+srv://USER:PASSWORD@jok-prod.RANDOM.mongodb.net/jok

And it connected to the jok database without specifying authSource (it used admin)

Maybe you are right and this change isn't correct, but than it means credentials.db shouldn't be used as authSource.

P.S. The main problem I want to fix is to use same connection string and make it work without setting authSource, always.

[playerx]

closing this PR and I'll continue checking what's the main issue