pull-job: make sure pull_job_restart() can be used to fetch the same resource again...

pull-job: make sure pull_job_restart() can be used to fetch the same resource again, just with new headers

Let's flush out all response state from the job, but let's keep the
request data previously configured, in particular the headers set. This
is useful to re-request a resource, just with a slightly modified or
identical URL.