Change EPP response code according to its specification

Fixes #587

test/integration/epp/login_test.rb

@@ -169,6 +169,6 @@ class EppLoginTest < EppTestCase
     assert_no_difference 'EppSession.count' do
       post '/epp/session/login', { frame: request_xml }, 'HTTP_COOKIE' => 'session=new-session-id'
     end
-    assert_epp_response :authentication_error_server_closing_connection
+    assert_epp_response :session_limit_exceeded_server_closing_connection
   end
 end