Use QUrl::toString() when forming the 'Host' header

This allows to correctly use IPv6 literals and also
deals (correctly) with a port (if it's set at all).

Task-number: QTBUG-68245
Change-Id: I6d29543887c4ab58d70f0970a6f0a1b822c301df
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>