Don't delete the pk of join table model

lib/associations/has-many.js

@@ -71,10 +71,6 @@ module.exports = (function() {
       if (this.hasJoinTableModel === true) {
         this.connectorDAO = this.options.joinTableModel
         this.connectorDAO.rawAttributes = Utils._.merge(this.connectorDAO.rawAttributes, combinedTableAttributes)
-
-        // FIXME - Should we do this? Normally join tables does not have an id, and it is not needed,
-        // but users might wonder what hapened if they defined the ID themselves
-        delete this.connectorDAO.rawAttributes.id
         this.connectorDAO.init(this.connectorDAO.daoFactoryManager)
       } else {
         this.connectorDAO = this.source.daoFactoryManager.sequelize.define(this.combinedName, combinedTableAttributes, this.options)