title: How to .update() value to NULL in sequelize
body: <p>I'm writing my service to update a row using sequelize for PostGres. When I try out my query using a PSequel it works fine:</p>
  
  <pre><code>UPDATE "test_table" SET "test_col"=NULL WHERE "id"= '2'
  </code></pre>
  
  <p>But using sequelize it throws a 500 error:</p>
  
  <pre><code>db.TestTable.update({ testCol: NULL }, { where: { id: id } })
    .then((count) =&gt; {
      if (count) {
        return count;
      }
    });
  </code></pre>
  
  <p>My model does allowNull which I believe is what allows null values to be the default as well as set:</p>
  
  <pre><code>testCol: {
    type: DataTypes.INTEGER,
    allowNull: true,
    defaultValue: null,
    field: 'test_col'
  },
  </code></pre>
  
  <p>Any other value but NULL works as expected. Is there a different method for setting null values?</p>