Allow initialization with explicit nil values

[jlecour]

The g gem is using ruby_gntp with explicit host/passwd params, so the GNTP.notify class methods is instanciating an object with nil values for both parameters, which makes Ruby's Resolv.create method raising an error : https://github.com/ruby/ruby/blob/ruby_2_0_0/lib/resolv.rb#L1169-L1178

By not giving null values to GNTP, it's using its default values.