unexpected error code for 'ResourceNotFoundError'

Bug Report

Restify Version

5.2.0

Node.js Version

7.10.1

Expected behaviour

// trigger this error when some in-existing route being called
server.on('NotFound', (req, res, err, cb) => {
  if (err instanceof errors.ResourceNotFoundError) {
    // should get here
  }

  cb();
});

Actual behaviour

// trigger this error when some in-existing route being called
server.on('NotFound', (req, res, err, cb) => {
  req.log.info(JSON.stringify(err, null, 2)); // { "code": "ResourceNotFound", "message": "/xxxx does not exist" }
  req.log.info(err.toString()); // ResourceNotFoundError: /xxxx does not exist
  req.log.info(err.code); // Error
  if (err instanceof errors.ResourceNotFoundError) {
    // never get here
  }

  cb();
});

Repro case

1. create a error handler for NotFound error (though I assume there should have a ResourceNotFoundError)
2. call API server with an in-existing route.

Start at the server.on('NotFound') handler and reproduce a request to an in-existing route with the reported Restify and Node.js versions. Trace where ResourceNotFoundError is created and how its code is assigned; done means the handler receives the expected error instance and behavior, with a regression test covering the repro.