Use correct method to extract image from vcard

It was incorrectly assumed, that $vcard is always an
OCA\Contacts\Contact (which is true when the contacts app is rendered)
but when called from the `contactUpdated` hook, it actually is a
OCA\Contacts\VObject\VCard.

Fixes owncloud/contacts#585
Merge pull request #598 from tribut/fix-585

Use correct method to extract image from vcard