"mpromise (mongoose's default promise library) is deprecated" error when testing
<p>For a while I have been getting this error and I decided to fix it today but after an hour trying to fix it I can find the solution.</p>

<p>When I test my mongoose User model this error/warning is generated:</p>

<pre><code>Mongoose: mpromise (mongoose's default promise library) is deprecated, plug in your own promise library instead: http://mongoosejs.com/docs/promises.html
</code></pre>

<p>Here is my test:</p>

<pre><code>  1 var assert = require('chai').assert;
  2 var mongoose = require('mongoose');
  3 var clearDB = require('mocha-mongoose')(require('../../config/database').uri, { skip: ['workouts'] });
  4 var database = require('../../config/database').connect;
  5
  6 var User = require('../../app/models/user');
  7 var user = new User({});
  8
  9 var req_body = {
 10   username: "garyvee",
 11   email: "gary@vaynermedia.com",
 12   password: "secret"
 13 };
 14
 15 describe('User', function() {
 16   beforeEach(function(done) {
 17     user.username = "johnsmith";
 18     user.email = "john@gmail.com";
 19     user.password = "secret";
 20     done();
 21   });
 22
 23   it('can be saved', function() {
 24     return user.save(function(err: any) {
 25       assert.isNull(err);
 26     })
 27   });
 28 });
</code></pre>

<p>I think it has something to do with the <code>.save</code> but I don't know how to fix it. Can someone help me and tell me how to fix it so that error/warning isn't shown.</p>