fix(belongs-to-many): ambigious id when through model has id

lib/associations/belongs-to-many.js

@@ -443,7 +443,7 @@ BelongsToMany.prototype.injectGetter = function(obj) {
 
     options = association.target.__optClone(options) || {};
     options.attributes = [
-      [sequelize.fn('COUNT', sequelize.col(model.primaryKeyAttribute)), 'count']
+      [sequelize.fn('COUNT', sequelize.col([association.target.name, model.primaryKeyAttribute].join('.'))), 'count']
     ];
     options.joinTableAttributes = [];
     options.raw = true;

test/integration/associations/belongs-to-many.test.js

@@ -423,6 +423,11 @@ describe(Support.getTestDialectTeaser('BelongsToMany'), function() {
         active: DataTypes.BOOLEAN
       });
       this.UserTask = this.sequelize.define('UserTask', {
+        id: {
+          type: DataTypes.INTEGER,
+          primaryKey: true,
+          autoIncrement: true
+        },
         started: {
           type: DataTypes.BOOLEAN,
           defaultValue: false