[BUGFIX] 9234 - WFS client doesn't expose Header Authorization

Like for the others OGC providers: WMS and WCS, the user can set the
login and password to access to WFS protected data. But the QGIS WFS
provider doen't expose header authorization. It doesn't use login and
password.

This patch set header authorization for all WFS connexion if login and
password are set.
And I think this patch has to be backported to release_2-2, because QGIS
can't acces to protected WFS data.
Merge pull request #1266 from rldhont/provider_wfs_authorization

[BUGFIX] 9234 - WFS client doesn't expose Header Authorization