title: sequelize update record primary key value
body: <p>I'm trying to update the id value of an existing instance using Sequelize but it is not being persisted to the DB</p>
  
  <pre><code>// model definition
  var User = sequelize.define('users', {
    id: {
      type: DataTypes.STRING,
      unique: true,
      primaryKey: true,
      field: 'id'
    }
  )};
  
  // usage
  User.findById(oldId).then(function(user) {
   user.id = 'new value';
   user.save(); // Id remains the same in DB
  });
  </code></pre>
  
  <p>I'm using sequelize 3.14.0 with node and a PostgreSQL database.</p>