enable JSON-dumping of NumericRange values

This is a bit of a hack to allow some more exotic values in model and
still properly relay them to the hosts.  We could end up with some
extents using NumericRange and others plain lists depending on how the
row have been loaded etc..

Proper solution would be to always receive JSONs from PostgreSQL.

Signed-off-by: Jan Dvořák <mordae@anilinux.org>