Fix request interceptor to process only urlBase requests

Signed-off-by: Ariel Schiavoni <arielschiavoni@gmail.com>
The code looks much better now. There are two things I'd like you to improve:

1. use a promise API instead of callback-based API, i.e. `promiseRequestHandler(promise).then(callback)`. I believe the trick is to change L60 from `callback(req)` to `return req`.

2. The name `promiseRequestHandler` does not describe the intent well enough. How about `expectMockedRequestPromise`?

With those two changes in place, the test code on L109 will become

```js
expectMockedRequestPromise(MyModel.count().$promise)
  .then(function(req) {
    expect(req.config.headers).to.have.property(authHeader);
  });