Add `nodejsHttpXAgent` member per class in addition to per instance

- before: `nodejsHttpXAgent` defined per `XMLHttpRequest` instance, making
  impossible to _stub_ HTTP proxies underneath an existing implementation
  (like `dropbox-je`)
- Now, per class `XMLHttpRequest.nodejsHttpXAgent`.  Per-instance value
  take the per-class by default & default to the `httpX.globalAgent` if
  the per-class value is not defined.
- I am new to coffee-script, so I have no idea on how to systematically
  test this.  It works here.