Add test for AR::Base#to_param

According to the doc of `AR::Base#to_param`(
https://github.com/rails/rails/blob/04cda1848cb847c2bdad0bfc12160dc8d554
7775/activerecord/lib/active_record/integration.rb#L18 ), it returns
`nil` if the record is not persisted.

Author: take

activerecord/test/cases/integration_test.rb

@@ -11,7 +11,12 @@ def test_to_param_should_return_string
     assert_kind_of String, Client.first.to_param
   end
 
-  def test_to_param_returns_id_even_if_not_persisted
+  def test_to_param_returns_nil_if_not_persisted
+    client = Client.new
+    assert_equal nil, client.to_param
+  end
+
+  def test_to_param_returns_id_if_not_persisted_but_id_is_set
     client = Client.new
     client.id = 1
     assert_equal '1', client.to_param