Girocco/Notify.pm: add json_test_post function
Rather than blindly accept just any old JSON POST url, the
json_test_post function can be called at the time the url
is first configured to verify that it can be POST'd to
successfully.
This is not unlike the behavior of that other well-known
service that we are emulating except that the intent is
to actually decline to configure the URL if anything other
than a "successful" (aka 2xx) status code comes back.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>