Pick vCard version by url parameters

[vcard-defaults-to-v4.patch.txt]( Describe the bug When downloading a VCF file from the CardDAV with the HTTP GET verb from a common web browser (Firefox, Chrome, Internet Explorer) one would expect to get a version 4.0 vCard, which is the best format supported by the CardDAV; nextcloud returns a version 3.0 vCard, downgrading the content. This is due to the fact that most browsers do not explicitly add "text/vcard; version=4.0" on their "Accept" HTTP header line; in place they use */*. Consequently, the negotiateVCard() function called by httpAfterGet() in the 3rdparty/sabre/dav/lib/CardDAV/Plugin.php script negotiates the content type as "text/vcard" then the text/vcard content type defaults to version 3.0. The same behavior is observed for a "Accept: text/vcard" header. To get a version 4.0 vCard, one has to set "Accept: text/vcard; version=4.0" explicitly in the header of the HTTP GET request. Actually, RFC…