title: Mongo: $match returns an empty array when is added to aggregate
body: <p>I have a query like this:</p>
  <pre><code>const data = await this.iveModel
        .aggregate([
          {
            $group: {
              _id: { $month: '$createdAt' },
              count: { $sum: 1 },
            },
          },
        ])
        .exec();
  </code></pre>
  <p>It works fine, but I want to limit the result to the current year, so I have added this:</p>
  <pre><code>const inicioDeAnio = moment()
        .startOf('year')
        .toISOString();
  
      const finDeAnio = moment()
        .endOf('year')
        .toISOString();
  
      const num = await this.iveModel
    .aggregate([
      {
        $match: {
          createdAt: {
              $gt: inicioDeAnio,
              $lt: finDeAnio
          }
      }
      },
      {
        $group: {
          _id: { $month: '$createdAt' },
          count: { $sum: 1 },
        },
      },
    ])
    .exec();
  </code></pre>
  <p>And this always returns an empty array.
  I do not know what I'm doing wrong.</p>