Add failing test for DS RR.
user
```ruby
require 'dns/zone/test_case'

class RR_DS_Test < DNS::Zone::TestCase

  TEST_DIGEST = '2BB183AF5F22588179A53B0A98631FAD1A292118'

  def test_build_rr__ds
    rr = DNS::Zone::RR::DS.new
    rr.label = 'dskey.example.com.'
    rr.key_tag = 60485
    rr.algorithm = 5
    rr.digest_type = 1
    rr.digest = TEST_DIGEST

    assert_equal "dskey.example.com. IN DS 60485 5 1 #{TEST_DIGEST}", rr.dump
  end

  def test_load_rr__ds
    rr = DNS::Zone::RR::DS.new.load("dskey.example.com. IN DS 60485 5 1 #{TEST_DIGEST}")
    assert_equal 'dskey.example.com.', rr.label
    assert_equal 'DS', rr.type
    assert_equal 60485, rr.key_tag
    assert_equal 5, rr.algorithm
    assert_equal 1, rr.digest_type
    assert_equal "dskey.example.com. IN DS 60485 5 1 #{TEST_DIGEST}", rr.key
  end

end
```
assistant