Allow initialization with explicit nil values

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.