Skip spurious .toLower() on returns of QUrl::scheme()

QUrl::setScheme() parses and canonicalises the scheme, so that
scheme() always returns a lower-case string anyway; no need to
.toLower() it.

Change-Id: Ied00814b63f159386a42552dcf06346ee56f9f97
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>