Auth method not negotiated automatically

In `Credential` interface it says that:
```
Which authentication mechanism to use. If not provided, one will be negotiated with the server.
```

But when one isn't provided, it throws:
```
MongoError: "Connection failed: MongoError: \"Auth mechanism not implemented in Deno driver: undefined\""
```

So this comment should either be removed or implemented.

Start at the Credential interface and reproduce the connection failure when no authentication mechanism is provided. Determine whether automatic negotiation is intended; done means either the documented negotiation works against the server or the misleading comment is removed, with the undefined-mechanism error no longer contradicting the interface.

Written by the indexing model from the issue text.