title: Sequelize "include" not working white many-to-many relations
body: <p>I'm new to Node and Sequelize, and i'm building a basic eCommerce.
  These are my <strong>models</strong>:</p>
  <pre><code>const Product = sequelize.define('product', {
    id: {
      type: DataTypes.INTEGER,
      autoIncrement: true,
      allowNull: false,
      primaryKey: true,
    },
    name: {
      type: DataTypes.STRING,
      allowNull: false,
    },
  });
  
  const Sale = sequelize.define('sale', {
    id: {
      type: DataTypes.INTEGER,
      autoIncrement: true,
      allowNull: false,
      primaryKey: true,
    },
    isDelivered: {
      type: DataTypes.BOOLEAN,
      defaultValue: false,
      allowNull: false,
    },
  });
  </code></pre>
  <p>And these the <strong>relations</strong>:</p>
  <pre><code>Sale.belongsToMany(Product, { through: SaleItem });
  Product.belongsToMany(Sale, { through: SaleItem });
  </code></pre>
  <p>The problem starts when I try to get the &quot;SaleItems&quot;</p>
  <pre><code> SaleItem.findAll({
      include: [Product],
    })
    .then(response =&gt; {
     console.log(response);
    })
  </code></pre>
  <p>I get the response:</p>
  <blockquote>
  <p>SequelizeEagerLoadingError: product is not associated to saleItem!</p>
  </blockquote>
  <p>It's strange, couse I'm doing the same on a one-to-many relation, and works perfectly.</p>
  <p>Thanks for your time :)</p>