_is_valid_netmask allows incorrect ipv4 masks

[GoogleCodeExporter]

>>> ipaddress.IPv4Network(0)._is_valid_netmask('128.128.128.128')
True

>>> ipaddress.IPv4Network(0)._is_valid_netmask('255.255.128.128')
True

each value different than 255 may be followed only by zeroes.

Original issue reported on code.google.com by tod...@gmail.com on 14 Dec 2012 at 6:34