Bugfix: resource.class.defaultprovider.ancestors.include breaks in spec ...

For whatever reason, this code breaks in Puppet RSpec tests:

      Failure/Error: it { should compile.with_all_deps }
     NoMethodError:
       undefined method `ancestors' for nil:NilClass
     # ./modules/vendor/sysctl/lib/puppet/type/sysctl.rb:34:in `default'

Hard-setting the default to Nil seems to do the trick though.