if no default provider specified use nil

    rspec-puppet gives an error otherwise

      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'
Merge pull request #20 from ddub/rspec-compatible

if no default provider specified use nil