Normalize case of our extensions

Summary:
Some backends (like my Nextcloud server) seem to change the case, making
our code not detect those anymore after a roundtrip. So fix that during
importing a vCard.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D20669