title: saslprep warning when using MongoClient.connect()
body: <p>I'm writing a NodeJS/express API and am facing the following warning when connecting to mongo server:</p>
  
  <pre><code>Warning: no saslprep library specified. Passwords will not be sanitized
  </code></pre>
  
  <p>Found no mention of this warning in documentation or github/google - is that OS (linux) library missing, or node package?</p>
  
  <p>This is the connection code sample:</p>
  
  <pre><code>const client = await MongoClient.connect(`mongodb://${auth[0]}:${auth[1]}@${url}/admin`, {
      useNewUrlParser: true
  });
  this.db = client.db(database);
  </code></pre>
  
  <p>How can I get rid of it?</p>
  
  <p><strong>Additional information:</strong></p>
  
  <p>Mongodb server: docker mongo:latest, resolved to 4.0.4 as of now</p>
  
  <p>mongodb library: 3.1.9</p>