vCard 2.1 behaviour

[da4089]

Import from: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=629898

VObject supports vCard 3.0 out of the box, although it is not
a standard yet. vCard 2.1 is (still) the standard and by far more
popular. Hence, albeit somewhat of a step backwards, it would be
good if VObject learnt the vCard 2.1 behaviour — and be able to
convert between them.