title: Mongoose save() doesn't return promise
body: <p>Mongoose's documentations affirms that method save returns a promise. I would like to save a user model in my database using this method. I do this like that :</p>
  
  <pre><code>save (user) {
      user.save((err, user) =&gt; {
          if (err) {
              return handleError(err);
          } else {
              console.log('The user ' + user.screenName + ' has been added.');
          }
  
          this.db.close();
      });
  }
  </code></pre>
  
  <p>Note that user param is :</p>
  
  <pre><code>mongoose.model('User', this.userSchema);
  </code></pre>
  
  <p>I don't understand why the console.log below returns false :</p>
  
  <pre><code>console.log((userRepository.save(user) instanceof Promise));
  </code></pre>
  
  <p>And of course when I try to then() the promise I doesn't work.</p>
  
  <p>Am I missing something ? I also tried to return the user.save(...) but it still did not work.</p>
  
  <p>Thanks for reading my message and have nice day!</p>