Problem Statement/Rationale

running follow command

```
mongoexport --host dbhost --port 27017 --db=mms-service --collection=document --type=csv --fields=_id --query='{}' --limit 10 --out exported.csv -u=user.name --authenticationDatabase admin
```

but i get an error

```
could not connect to server: connection() error occured during connection handshake: handshake failure: username must contain a '.' separated database.user pair
```

what is the meaning of the error, it is the issue of the username, a dot in the username?

i also try to add quote to the username: "user.name", 'user.name', same error

Steps to Reproduce

use mongo/mongoexport to connect to the db

Expected Results

login the db successfully

Actual Results

login failed

Additional Notes

1, also try mongo command, same error
```
mongo "mongodb://user.name:password@db-host:27017/?authSource=admin"

mongo --host dbhost --port 27017 -u user.name -p password --authenticationDatabase admin
```
error message:
```
MongoDB shell version v4.4.13
connecting to: mongodb://dbhost:27017/authSource=admin&compressors=disabled&gssapiServiceName=mongodb
Error: username must contain a '.' separated database.user pair :
connect@src/mongo/shell/mongo.js:374:17
@(connect):2:6
exception: connect failed
exiting with code 1
```

2, connect the db without auth, then auth by db.auth(), in this way, i can login into the db
```
mongo --host dbhost -p 27017
db.auth("user.name", "password") – the quote required
user dbname
show collections
```
it show the collections.