add additinalHeaders option for HTTPTask

One example of use case is sending request with a HTTP Basic Auth header instead of waiting for a challenge, which may not work for the case like server response different result for auth users and non-auth users.

I checked out the tests. They are not working. So I didn't spend time to write a test for this change.
As this change is simply just pass an config to session config, and works for myself, so just send this pull request.