chg: dev: make it easier to recycle the Requests Session
chg: dev: make it easier to recycle the Requests Session

In some cases, such as when operating inside a project which has
fork()-ed, it is necessary to close the existing session and reconnect.
This change make that possible simply by resetting the session attribute
to None and allowing it to be recreated.
toastdriven/pysolr,toastdriven/pysolr,django-haystack/pysolr,django-haystack/pysolr