t/00-startup: fix updateclass test

We called MogileFS::Client::update_class incorrectly without
the key.  Additionally, the test for checking the number of
copies was also incorrect.
support updating the class to the default class which has an id of 0

[ew: added trivial test]
Signed-off-by: Eric Wong <normalperson@yhbt.net>
add a hook to cmd_updateclass

Signed-off-by: Eric Wong <normalperson@yhbt.net>