don't use undefined in default options but empty objects

lib/model-factory.js

@@ -8,9 +8,9 @@ module.exports = (function() {
 
     this.options = Utils._.extend({
       timestamps: true,
-      instanceMethods: undefined,
-      classMethods: undefined,
-      validate: undefined,
+      instanceMethods: {},
+      classMethods: {},
+      validate: {},
       freezeTableName: false,
       underscored: false,
       paranoid: false