Improve object create/destroy : vcard

[mokraemer]

If you implement Carddav and your storage does not contain the vcard, you create the vcard with
$tmp['carddata']=(\VObject\Component\VCard())->serialize();
Next in Plugin.php the card is unserialized and "converted".

It would be better to allow objects here as well, and do the serialization at this point. If it is required to always check the output format, even it is stored in the db, it would be best, to require carddata to be an VCard object.