Set ConnectInBackground based on request

Set ConnectInBackground based on request

Set the QNetworkSession attribute to match the QNetworkRequest that
triggered it.
Currently if there are a mix of normal and background requests queued
and background connections are not allowed by policy:
 - background requests at the head of the queue should fail
 - first foreground request should successfully start a connection
 - remaining requests succeed regardless of background attribute

Change-Id: If0e3ec0b8a5096e3d7cd6df85884c6f53172d233
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>