The default properties defined in the parent class are not being inherited by its subclassed error types

[PabloRegen]

Here's a testcase:

var createError = require('create-error');

var MyCustomError = createError('MyCustomError', {prop1: 1});
var SubCustomError = createError(MyCustomError, 'CoolSubError', {prop2: 2});
var sub = new SubCustomError('My Message', {prop3: 3});

console.log(sub.prop1); // undefined -> should be 1
console.log(sub.prop2); // 2
console.log(sub.prop3); // 3