title: sequelize group by with multiple fields results into sequelize error
body: <p>My query is as shown below :</p>
  
  <pre><code> db.test.findAll({
              group: ['source'],
              attributes: ['source','source_id', [Sequelize.fn('COUNT', 'source'), 'count']],
              order: [[Sequelize.literal('count'), 'DESC']],
              raw: true
          }).then((sources) =&gt; {
              console.log('source is ' + sources);
          }).catch((err) =&gt; {
              console.log('error i is ' + err);
          })
  </code></pre>
  
  <p>test.js</p>
  
  <pre><code>module.exports = function (sequelize, DataTypes) {
      const test = sequelize.define('test', {
          id: {primaryKey: true, type: DataTypes.INTEGER, autoIncrement: true, allowNull: false, unique: true},
          source: {type: DataTypes.STRING, allowNull: false},
          source_id: {type: DataTypes.STRING, allowNull: false},
  
      });
  
      return test
  };
  </code></pre>
  
  <p>As soon as I run this query , it gives me this error :</p>
  
  <p><code>Expression #2 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'db.test.source_id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by</code></p>