Require attribute

#660

test/fixtures/nameservers.yml

@@ -0,0 +1,3 @@
+ns1:
+  hostname: ns1.bestnames.test
+  domain: shop

test/models/nameserver_test.rb

@@ -0,0 +1,17 @@
+require 'test_helper'
+
+class NameserverTest < ActiveSupport::TestCase
+  def setup
+    @nameserver = nameservers(:ns1)
+  end
+
+  def test_valid
+    assert @nameserver.valid?
+  end
+
+  def test_invalid_without_domain
+    @nameserver.domain = nil
+    @nameserver.validate
+    assert @nameserver.invalid?
+  end
+end