Infinite recursion(no pun intended) with BillingInfo.account_code

[bradodarb]

When serializing an instance of BillingInfo, a RangeError will be encountered due to the following code:
https://github.com/ceejbot/recurring/blob/master/lib/models/billing-info.js#L53

The class is creating a property getter that points to itself.

An error similar to the following is produced:

"err": {
    "message": "Maximum call stack size exceeded",
    "name": "RangeError",
    "stack": "RangeError: Maximum call stack size exceeded\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:52:43)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)\n    at BillingInfo.__defineGetter__ [as account_code] (/var/task/node_modules/recurring/lib/models/billing-info.js:53:19)"
  }

Looks like

 this.__defineGetter__('account_code', () => {
      return this.account_code
    })

Should be

 this.__defineGetter__('account_code', () => {
      return this.properties.account_code
    })

Happy to send a PR if this repo is still active.