Make QUrl always lowercase the scheme().

Make QUrl always lowercase the scheme().

URL schemes can only contain alphanumeric characters and all
protocols specify that they are case-insensitive. So instead
of doing case-insensitive comparison everywhere and then get
it wrong sometimes, better to lower-case it here.

Change-Id: I61f51a3f4c85b90af1586ebcf69608987fbe2ec3
Reviewed-by: Jonas Gastal <jgastal@profusion.mobi>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>