Set a filename with a .vcf extension when exporting address books
Set a filename with a .vcf extension when exporting address books

Filename format is <addressbookname>-YYYY-MM-DD.vcf. Content type used
to be text/directory, but according to RFC 6350 that mimetype is
deprecated and text/vcard should be used instead, so this commit also
changes that.

fruux/sabre-dav,c0d3z3r0/sabre-dav,kriswebdev/sabre-dav,c0d3z3r0/sabre-dav,kriswebdev/sabre-dav,fruux/sabre-dav,c0d3z3r0/sabre-dav,kriswebdev/sabre-dav,fruux/sabre-dav,PVince81/sabre-dav,PVince81/sabre-dav,staabm/sabre-dav,PVince81/sabre-dav,staabm/sabre-dav,staabm/sabre-dav