Testing express middleware that returns next() / next("route")

I created this piece of middleware that will return next() when the only parameters for a route are defined by queryItems.

I recently found node-mocks-http which fakes expresses req and res objects. However it does not fake next. I was wondering how that should be done? Below I have an example of me opening up the next callback and defining my expect statement within it.

middleware.hasOnlyQuery = function(queryItems){
  return function(req, res, next){
    if(typeof queryItems == "string") queryItems = [queryItems]
    if(_.hasOnly(req.query, queryItems)) return next()
    return next("route")
  }
}

Here's the test.

it("should only have shop query", function(done){
  var req = httpMocks.createRequest({
      method: 'GET',
      query: {
        foo: "bar"
      }
  });
  var res = httpMocks.createResponse()
  var fn = middleware.hasOnlyQuery(["foo"])(req, res, function(err){
    expect(err).to.equal()
    return done()
  })
})

it("should not only have shop query", function(done){
  var req = httpMocks.createRequest({
      method: 'GET',
      query: {
        foo: "bar",
        bar: "foo"
      }
  });
  var res = httpMocks.createResponse()
  var fn = middleware.hasOnlyQuery(["foo"])(req, res, function(err){
    expect(err).to.equal("route")
    return done()
  })
})

Is this the proper way to do this? Is there any way to make this simpler / easier, perhaps converting it to a promise so I can use chai-as-promised?

Note: _.hasOnly is a custom underscore mixin.