title: Sequelize is returning integer as string
body: <p>I using nodejs v4 with sequelize, and I have a model like this:</p>
  
  <pre><code>var Device = sequelize.define('Device', {
  id: {
    type: DataTypes.BIGINT,
    primaryKey: true,
    autoIncrement: true
  },
  tenantId: {
    type: DataTypes.BIGINT,
    allowNull: false
  },
  token: {
    type: DataTypes.STRING,
    allowNull: false
  }
  }, {
   tableName: 'devices'
  });
  </code></pre>
  
  <p>When I select a device by id the type of id is a string, exemple:</p>
  
  <pre><code>Device.findById(9).then( function(result) {
    console.log(result.toJSON().id + 10);
  });
  </code></pre>
  
  <p>The output will be 910, rather than 19, so I look at json and a saw this:</p>
  
  <pre><code>{
    id: "9"
    tenantId: "123"
    token: "adsadsdsa"
  }
  </code></pre>
  
  <p>The id in found device is a string, but I defined it as a number...</p>
  
  <p>Doesn't it should be { "id": 9 } ?</p>
  
  <p>How can I select a device with the types that I defined previously?</p>