Add Registrant API contact update action

Closes #849

test/models/contact/contact_test.rb

@@ -26,4 +26,16 @@ class ContactTest < ActiveSupport::TestCase
   def test_not_in_use_if_acts_as_neither_registrant_nor_domain_contact
     refute contacts(:not_in_use).in_use?
   end
+
+  def test_managed_when_identity_codes_match
+    contact = Contact.new(ident: '1234')
+    user = RegistrantUser.new(registrant_ident: 'US-1234')
+    assert contact.managed_by?(user)
+  end
+
+  def test_unmanaged_when_identity_codes_do_not_match
+    contact = Contact.new(ident: '1234')
+    user = RegistrantUser.new(registrant_ident: 'US-12345')
+    assert_not contact.managed_by?(user)
+  end
 end