QNetwork: Check AuthenticationReuseAttribute when using credentials from url

Note: This is not needed in Qt5, as the authenticationRequired method in Qt5
has an allowAuthenticationReuse parameter (bool) that is checked before
the credentials are used.

Change-Id: I5a2734de615a1a96d1fe648bd251850f3b45e167
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>