fixes #1432 - subnet gateway infos only saved when fill gateway and dns fields

the informations in the subnet form only saved if you fill the gateway and one dns fields
  the cause is the validation in models/subnets.rb

whyever the :allow_blank should work, but i need to add :allow_nil also to get this working

Signed-off-by: Florian Koch <florian.koch1981@googlemail.com>