title: set field as empty for mongo object using mongoose
body: <p>I'm calling user.save() on an object, where I set user.signup_date = null;</p>
  
  <pre><code>user.first_name = null;
  user.signup_date = null;
  
  user.save();
  </code></pre>
  
  <p>But when I look at the user in the mongodb it still has the signup_date and first_name set...how do I effectively set this field as empty or null?</p>