add .originalUrl about koa

[jonathanong]

going to not touch req.url and instead save any changes to the URL as this.request._url. we can then do:

this.request.__defineGetter__('originalUrl', function () {
  return this.req.url;
})

what do you think?

[tj]

I think it's best to still modify node's req.url so it reflected within any non-koa-ish libraries properly