Fix performance problem with NTLM auth in http POST

Fix performance problem with NTLM auth in http POST

A combination of other fixes resulted in authentication credentials
with a blank username & password being incorrectly added to the
cache only for NTLM.
This caused authentication to be attempted with blank credentials
first (which would fail), before the authenticationRequired
signal was emitted.
It caused a performance issue because of the extra 2 requests to the
server (and possibly delays inserted by the server following a failed
login)

Change-Id: Ic588a20cfe7c24d5e60cd384caff0673a587e484
Reviewed-by: Martin Petersson <Martin.Petersson@nokia.com>